Pressure Oxidation of Molybdenum Concentrates

Transcription

1 Colorado School of Mines Pressure Oxidation of Molybdenum Concentrates MME Senior Design Project Spring 2013 Kyle Gough, Sarah Holmes, Neal Matosky, Tiffani Oney, Jordan Rutledge 4/23/2013 Submitted to: Dr. Corby Anderson

2 2 Table of Contents Figures and Tables... 3 Executive Summary Introduction Background Roasting Scope Literature Survey Concentrate Analysis Experimental Method Experimental Results and Discussion Tables of Recoveries Stat-Ease Optimizations Process Design Criteria Flowsheet Economics Conclusions Appendices Appendix 1: Molybdenum Concentrate Roasting Contracts Appendix 2: Robinson Mine Daily Production Summary Appendix 3: Robinson Mine Flowsheet Appendix 4: Mass Balance for Alkaline System Appendix 5: Stat Ease Model for Acidic System Appendix 6: Stat Ease Model for Alkaline System Appendix 7: Design Criteria Calculations Appendix 8: Economics References... 82

3 3 Figures and Tables Figure 1. Mineral liberation analysis of molybdenum concentrate [1] Figure 2. Multiple hearth roasting furnace placed in a molybdenum roasting plant [3] Figure 3. Eh-pH diagram for molybdenum [6] Figure 4. Eh-pH diagram for rhenium [6] Figure 5. Eh-pH diagram for copper in chalcopyrite form at 25 C and 1 atmosphere total pressure [7] Figure 6. Particle size versus percent cumulative passing Figure 7. Mineral liberation analysis of molybdenum concentrate [1] Figure 8. Linear regression model to determine approximate stirring speed Figure 9. Response surface model for molybdenum recovery from the acidic system Figure 10. Response surface model for rhenium recovery from the acidic system Figure 11. Response surface model for copper recovery from the acidic system Figure 12. Response surface model for molybdenum recovery from the alkaline system Figure 13. Response surface model for rhenium recovery from the alkaline system Figure 14. Design criteria for Robinson Mine acidic pressure oxidation system Figure 15. Flowsheet for the acidic pressure oxidation leaching of the molybdenum concentrate Figure 16. NPV sensitivity, showing that revenue per ton molybdenum concentrate treated has the greatest effect on NPV Table 1. Robinson Mine concentrate [1] Table 2. Production summary of Robinson Mine [2] Table 3. Particle size distribution of molybdenum concentrate for representative sample of 200 g Table 4. Robinson Mine concentrate [1] Table 5. Design matrix used for experimental procedure Table 6. The recoveries of molybdenum, rhenium, and copper from acidic pressure leaching, considering the experimental factors of grind time, solids concentration, initial acidity, temperature, and time [1] Table 7. The recoveries of molybdenum, rhenium, and copper from alkaline pressure leaching, considering the experimental factors of time, temperature, solids concentration, and NaOH concentration Table 8. Summary of economics... 34

4 4 Executive Summary Nearly all molybdenum concentrates are refined through pyrometallurgy as it is a well-defined process that produces desirable results. However, due to the copper penalties and low-grade molybdenum content of the studied molybdenum concentrate from the Robinson Mine of KGHM Polska Miedz (37.5% molybdenum, 1.59% copper), pyrometallurgy is not a viable option. The global roasting facilities will not accept it and they will not pay for the rhenium content. The significant presence (0.159%) of rhenium, a high-priced metal used in nickelsuperalloys, serves as major motivation for processing of this concentrate. Pressure oxidation offers an alternative to a pyrometallurgical circuit by extracting molybdenum and rhenium from the concentrate without penalization due to the presence of copper. This study compares and evaluates the economic feasibility and payback of both alkaline and acidic pressure oxidation systems. Following limited experimentation, the alkaline process proved to be less efficient than the acidic process. At optimum temperature and time parameters the alkaline leach recovers only half that of optimum recovery of the previous study involving acidic leaching. However, with development of current practices and equipment, alkaline pressure oxidation still shows some potential for selective leaching. Moving forward with design, it is currently recommended to use acidic pressure oxidation leaching to extract molybdenum and rhenium from the Robinson Mine concentrate. This paper assesses the potential of alkaline pressure oxidation, evaluates and compares the economic feasibility of alkaline versus acidic pressure oxidation, and outlines design criteria for acidic pressure oxidation in application to the Robinson Mine. In summary, the pressure oxidation process chosen appears technically viable. As well, a capital investment of about $ 7 M USD to build a plant to treat the Robinson concentrate would offer a before tax payback period of under one year with an NPV of about $ 45 M USD at an 8 % discount rate and IRR of over 100 %. Qualifying Statement This report was prepared for the KGHM Polska Miedz Robinson Mine and is based on information available at the time of the report preparation. It is believed the information, estimates, conclusions and recommendations contained herein are reliable under the conditions and subject to the qualifications set forth. Furthermore, the information, estimates, conclusions and recommendations are based on the experience of the noted MME Senior Design Group and data supplied by others, but the actual result of the work is dependent, in part, on factors over which the noted MME Senior Design Group has no control. This report is intended to be used exclusively by KGHM Polska Miedz s Robinson Mine. Any other use of or reliance on this report is at the sole risk of the party that so relies.

5 5 SCOPE OF WORK Client Name: KGHM Polska Miedz - Robinson Mine, Dr. Corby Anderson Project Name: Pressure Oxidation of Molybdenum Concentrates 1.0 Introduction The purpose of this study is to initially evaluate and compare an acidic versus alkaline system for the pressure oxidation of molybdenum concentrate for the benefit of the Robinson Mine. Generally pyrometallurgical methods to treat molybdenum concentrates require multiple steps of processing; however, pressure oxidation offers a more direct treatment of the concentrate and thus a potentially more economically viable option. Experimentation and analysis for the acidic system was completed by a previous team under the lead of Dr. Corby Anderson. The alkaline system was tested under similar conditions by the 2013 Senior Design Team within Colorado School of Mines laboratories. 1.1 Background The specific molybdenum concentrate was from KGHM Polska Miedz s Robinson Mine in Ely, Nevada. The concentrate elemental analysis and quantitative mineral phase analysis by mineral liberation analysis are shown in Table 1 and Figure 1: Table 1. Robinson Mine concentrate [1]. Mo, % Cu, % Re, % Fe, % TS, % TC, %

6 6 Figure 1. Mineral liberation analysis of molybdenum concentrate [1]. Typical contractually required values of a molybdenum concentrate are 50+ % molybdenum and < 1% copper 1. Therefore, this is an off-grade molybdenum concentrate with substantial rhenium credits that is not acceptable for roasting. Although the molybdenum concentrate of the Robinson Mine deposit contains low-grade copper and molybdenum, rhenium s high price and high-grade presence in the concentrate served as a major motivation in considering a pressure oxidation process. Currently, smelting costs to refine the low-grade molybdenum and high-grade rhenium Robinson Mine molybdenum concentrate greatly outweigh the economic value of finding a 1 Appendix 1: Molybdenum Concentrate Roasting Contracts

7 7 roaster to take the concentrate. From a review of the Robinson Mine production data 2, it appeared that molybdenum and, likely, rhenium recovery is being reduced in the flotation plant in order to produce an acceptable grade of molybdenum concentrate. Therefore, a pressure oxidation system was considered as a potential alternative to smelting for the economic benefit of KGHM Polska Miedz and the Robinson Mine. Acidic pressure oxidation is a more common process than alkaline for molybdenum refining. A key distinction is that copper is soluble only in an acidic leach solution. However, rhenium and molybdenum are soluble in an alkaline environment and can therefore be recovered selectively from a concentrate via an alkaline pressure oxidation system. As the price of rhenium is currently high, a successful pressure leaching system could offer substantial economic payback. Currently, the Robinson Mine produces a molybdenum flotation concentrate as a secondary product to copper 3. Flotation is also used to produce a primary copper and gold concentrate. Table 2 displays the production summary for Table 2. Production summary of Robinson Mine [2] Cu Production (M lbs) Cu Production (Tonnes) 53,407 47,244 43,137 Au Production (k oz) Waste Mined (Mt) Ore Milled (Mt) Cu Grade (%) Au Grade (g/t) Cu Recovery (%) Au Recovery (%) See Appendix 2: Robinson Mine Daily Production Summary 3 See Appendix 3: Robinson Mine Flowsheet.

8 8 1.2 Roasting Pyrometallurgy remains as the primary means by which most metals are processed metallurgically. Included in this method of treating metals is roasting, a process that involves a gas-solid reaction where solid is introduced to high-temperature environments in order to further purify the desired components. Nearly all molybdenum concentrates today are subjected to a roasting process, as it is a well-defined process that gives desirable results. Multiple options of furnaces exist that all accomplish the desired goal of roasting of molybdenum concentrates. However, the most widely used piece of equipment is the multiplehearth furnace, shown in Figure 2. Figure 2. Multiple hearth roasting furnace placed in a molybdenum roasting plant [3]. Molybdenum concentrate is fed from the roaster feed bin to the belt feeder, where discharge occurs into the screw conveyor following which the concentrate is fed into the roaster. Multiple hearth roasters have 12 hearths, each of which is air-cooled. Rabble blades rake the material until

9 9 it drops to the next hearth down the furnace. Between hearths 3 to 9 most of the sulfur of the MoS 2 is eliminated and in the last three hearths the remaining sulfur is removed, yielding molybdenum trioxide (MoO 3 ). Rhenium is collected as a dust and further refined from rhenium heptoxide (Re 2 O 7 ). Regardless of the consistency provided by multiple hearth furnaces in yielding quality product there are several limitations that raise the question of whether or not pyrometallurgy is the most ideal process by which to process molybdenum concentrate. Primarily, the escape of dust and creation of sulfur dioxide are potential environmental concerns that can be mediated by the substitution by a hydrometallurgical process, such as pressure oxidation. As well, pyrometallurgical rhenium production is not direct. 2.0 Scope The use of pressure oxidation to leach a metal is a relatively new process and uses lower temperatures than pyrometallurgical extraction techniques. Pressure oxidation uses either an acidic or alkaline leaching system driven by elevated temperature, steam pressure, and oxygen in order to induce a reaction. The pressure within the autoclave is elevated from steam pressure and as oxygen is added to the closed container. As the solution is stirred, an environment is created that can be controlled to effectively leach the desired material [4]. This process has been used by many companies including Rio Tinto, ASARCO, and Freeport McMoRan [5]. Different metals can be leached out depending on the conditions in the autoclave, specifically focusing on whether it is an acidic or alkaline environment. The decision to choose an acidic or alkaline leaching is primarily dependent on the desired material to be recovered. For example, silver and gold concentrates are both generally treated by acidic leaching, while alkaline leaching

10 10 tends to be used for aluminum production via the Bayer Process. The alkaline pressure oxidation process was chosen for the concentrate specified in this report based on Eh-pH diagrams and a desire to determine selectivity of molybdenum and rhenium over copper. The Eh-pH diagrams may found in Figures 3, 4, & 5. Based on these diagrams, it is evident that molybdenum and rhenium are both soluble at high ph values, while copper remains insoluble. This allows molybdenum and rhenium to be leached into solution for recovery, while copper remains in the solid. Figure 3. Eh-pH diagram for molybdenum [6]. Figure 4. Eh-pH diagram for rhenium [6].

11 11 Figure 5. Eh-pH diagram for copper in chalcopyrite form at 25 C and 1 atmosphere total pressure [7]. Currently, the demand for molybdenum and rhenium is very high. Molybdenum is primarily used as a hardening mechanism in steel. As of January 2013, molybdenum was valued at $11.84 per pound, compared to common copper priced at only $3.65 per pound. However, rhenium is the most valuable of the three at $2, per pound [8] as it is used in products such as in nickel-based superalloys, making it an important and rare metal. Pressure oxidation has key design parameters that can be changed and controlled in order to extract the metals efficiently. The parameters focused on in this study are temperature, leaching time, solids concentration, and the concentration of either acidic or alkaline leach solution (in the

12 12 previous acidic study concentrate grinding was also considered). Other parameters that can be changed include pressure and stirring speed. If the process is executed for a longer time, the recovery is normally better. By changing the concentration of the solids, the solution will become more or less saturated in the autoclave. Lastly, stirring speed is dependent on the autoclave s design. If the speed is too fast, the lining of the autoclave may be worn down by rapidly moving solid particles. However, too slow of a stirring speed and the solids will remain at the bottom of the autoclave where much of the material will remain unreacted. 3.0 Literature Survey Commonly, treatment of molybdenum concentrates has been a multiple-step process involving roasting, whereas pressure oxidation is direct leaching of the desired material into solution. Although it is not as common applied of a process, pressure oxidation has the potential to effectively treat off-grade concentrate. In addition, many environmental restraints have become progressively stricter for pyrometallurgical processes; this further decreases the feasibility of using roasting for an off-grade concentrate. The use of hydrometallurgy, specifically pressure oxidation, has shown promise in several recent studies for the treatment of molybdenum concentrates [9]. The use of hydrometallurgical methods to recover metal from concentrates is three-fold: the first step being the dissolution of metal, the second step being the purification of the leaching solution, and the third step being the recovery from the purified solution. Although many reactions are often thermodynamically favorable, often they are not kinetically feasible consequently promoting the use of increased temperature and pressure. Increasing temperature greatly increases the reaction rate of a system which in turn allows for faster production. The

13 13 first and most well-known hydrometallurgical pressure operation is the Bayer process in which alumina is removed from bauxite [3]. A study completed by Kaixi Jiang from the Beijing General Research Institute of Mining and Metallurgy outlines the benefits of using a pressure oxidation unit rather than the traditional roasting circuit. Sulfur dioxide (SO 2 ) pollution was greatly reduced by the pressure oxidation method. While the study outlines the pressure oxidation of different non-ferrous elements such as gold, nickel and aluminum, the main focus remains on molybdenum concentrate. The concentrate used in the study was 44.95% molybdenum and 0.067% rhenium. The particle size analysis revealed that 91.77% of the concentrate was smaller than 200 mesh. The use of nitric acid in the pressure oxidation process was shown to help improve the reaction efficiency and decrease the necessary reaction time and temperature. A concentrate from the Baoshan Copper Mine in China showed promising results with the use of nitric acid; however, the production of nitric oxide and nitrogen dioxide posed many safety problems. This study prefers the use of an alkaline system using either sodium hydroxide (NaOH) or sodium carbonate (Na 2 CO 3 ) as the leaching agent opposed to an acidic system utilizing sulfuric acid. Alkaline leaching requires lower reaction temperatures and operation pressures and in general has lower equipment costs than acidic systems. However, the high production cost in consumption of NaOH often limits the use of alkaline systems. The reaction equations for the alkaline and acidic leach systems are shown below: Alkaline: MoS 2 + 6NaOH + 9/2O 2 = Na 2 MoO 4 + 2Na 2 SO 4 + 3H 2 O 2ReS NaOH + 19/2O 2 = 2NaReO 4 + 4Na 2 SO 4 + 5H 2 O

14 14 Acidic: MoS 2 + 3H 2 O + 9/2O 2 = H 2 MoO 4 + 2H 2 SO 4 4ReS O H 2 O = 4HReO 4 + 8H 2 SO 4 Temperature, reaction time, and pressure proved to be influential factors in the alkaline system. Temperature was by far the most important variable within the system with an increase in temperature from 150ºC to 230ºC, the molybdenum oxidation increased from 23% to 98.75%. With the optimum operating conditions of 4 hours, 1.6MPa, and 180ºC, 99% of the molybdenum was oxidized and the leaching ratio for rhenium was around 95% [10]. In the study done by Mirvaliev and Inoue, two types of molybdenum concentrates were tested under acidic pressure oxidation conditions: low-grade (approximately 21% molybdenum) and high-grade (99.6% molybdenum). Experiments were performed in a 200 ml autoclave and a variation of rpm stirring rate and 1-10 g of concentrate in ml of solution. A range of g/l sulfuric acid was used for the leach. Molybdenum and a substantial amount of iron were extracted into the leach solution. The solution increased in ph approximately 20 minutes into leaching. Recovery of this concentrate can be achieved by flotation or oxidative leaching in alkaline solution. Mirvaliev and Inoue also experimented with an alkaline leach solution with the same concentrates. Alkaline agents used were NaOH, Na 2 CO 3, and ammonium hydroxide (NH 4 OH). For leaching times less than one hour, Na 2 CO 3 provided the best recovery of molybdenum, while NaOH was the better choice for longer leaching times. The ammonia solution provided a better recovery of molybdenum in higher copper concentrations. NaOH was more favorable in concentrates with fewer impurities. The alkaline solution can yield between 40-95% molybdenum recovery depending on alkaline concentration [11].

15 15 The Kennecott Copper operations, owned by Rio Tinto, have also experimented with pressure oxidation technology for molybdenum concentrate. The typical concentrate suggested for this type of treatment should contain an economically significant amount of molybdenum disulfide (usually at least 20%, but no higher than 50%), particle size of less than 100 mesh (U.S. Standard), and not include previously processed materials. The latter is due to an excess of gangue materials which cause the process to be less efficient. The particle size (P 80 ) was preferably at 200 or finer mesh to facilitate the oxidation step. The concentrate was between 5-40 or weight percent. The concentrate was slurried with water and an aqueous solution of metal salts and/or acid. Temperature varied between C but ranged mostly around 150 C. Partial pressure of oxygen was also controlled from psi. The oxygen could have been introduced as pure oxygen or oxygen enriched air. Timing of the reaction varied, but proceeded to completion at minimum 90% recovery. The parameters manipulated to selectively recover molybdenum and rhenium within the pregnant liquor were temperature, oxygen pressure, and weight percent solids added. The product of the oxidation reaction yielded soluble molybdic oxide, insoluble molybdenum trioxide, and other soluble metal sulfates [12]. The study by Wangzhong Mu in the Applied Mechanics and Materials journal outlined the transformation behavior for a Chilean molybdenum concentrate undergoing pressure oxidation. The concentrate was composed of 48.1% molybdenum, 0.10% copper, and 0.002% rhenium. The argument for the use of pressure oxidation over roasting of molybdenum concentrate was validated by a desire for reduction in environmental pollution and energy consumption. The transformation rate of molybdenum and leach rate were both monitored in the study. By the addition of sodium nitrate into the system the leaching rate was increased substantially. Raising the temperature from 110ºC to 150ºC increased the total transform rate and absolute transform

16 16 rate, while slightly slowing the leach rate. When increasing the acid concentration from 20 to 40 g/l, all three transformation rates increased. With a concentration of 1:4 of NaNO 3, 40g/L sulfuric acid, 150ºC temperature, two hour reaction time, 0.8MPa oxygen partial pressure, and liquid solid ration of 6:1, the total transformation of molybdenum reached 97.95% [13]. With increasing energy costs and environmental regulations, alternative methods to feasibly recover metals from molybdenum concentrates has been intensively researched. There are few known rhenium deposits, thus the majority of the rhenium is produced as a byproduct of molybdenum-copper concentrates [14]. The concentrate used in this study is an excellent candidate for pressure oxidation method due to the low-grade molybdenum but substantial rhenium credits. The previous work outlined in this literature review help to provide guidelines for the design of a successful pressure oxidation study. 4.0 Concentrate Analysis This report specifically compares the use of alkaline versus acidic pressure oxidation for the recovery of molybdenum and rhenium, however all data for acidic leaching was taken from a previous publication prepared by Dr. Corby Anderson [1]. Therefore, this procedure only outlines the experimental process for alkaline leaching. The particle size distribution of the molybdenum concentrate was determined using sieve analysis. In order to ensure a representative sample was used, a Jones splitter was employed until a sample of approximately 200 g was achieved (total of five splits). After collecting a representative sample, the particle size distribution was found by sieve analysis using a ROTAP. The collected data is presented in Table 3 and plotted in Figure 6. Also, the MLA and elemental analysis are again presented as Figure 7 and Table 4.

17 17 Table 3. Particle size distribution of molybdenum concentrate for representative sample of 200 g. U.S. Standard Mesh Size Sieve Size (μm) Mass Collected in Sieve (g) % Mass Passing (Cumulative) Remainder Total mass = Figure 6. Particle size versus percent cumulative passing.

18 18 Figure 7. Mineral liberation analysis of molybdenum concentrate [1]. Table 4. Robinson Mine concentrate [1]. Mo, % Cu, % Re, % Fe, % TS, % TC, % Experimental Method The variables under consideration for alkaline pressure oxidation testing included time ( minutes), temperature ( C), solids concentration (10-30g/L), and NaOH concentration (20-40g/L). With four factors being examined, a full 2 4 factorial design was chosen with three center points, totaling to 19 tests. Table 5 contains the experimental matrix. Once again, it is

19 19 important to note that oxygen pressure and stirring speed in the autoclave were kept constant at approximately 100 psig and 500 rpm, respectively. An Autoclave Engineers 2-Liter Titanium Grade 2 autoclave was used for the pressure oxidation experiments, following the procedure designed by Kimberly Conner in her Ph.D. thesis [15]. Because this autoclave does not specifically designate stirring speed in rpm, it was necessary to determine the manual percent output level through a linear regression model, displayed in Figure 8. Therefore, the stirring speed was kept constant at a manual percent output level of 29%, or approximately 500 rpm.

20 20 Table 5. Design matrix used for experimental procedure. Std. Run Factor A: Time (min) Factor B: Temperature ( C) Factor C: Solids Concentration (g/l) Factor D: NaOH Concentration (g/l)

21 21 Figure 8. Linear regression model to determine approximate stirring speed. To perform a run, one liter of alkaline leach feed solution was prepared based on the NaOH concentration in the experimental matrix. Then, the mass of molybdenum concentrate specified by the matrix was spilt and weighed out based on testing with one liter leach feed solution. At this point, the autoclave was charged with the alkaline leach feed solution and preheated to approximately 90 C. Upon reaching this temperature, the molybdenum concentrate sample was added to the solution and the autoclave was sealed. With the cooling water on, the agitator was set and oxygen added to the system. The autoclave was then run at the designated temperature and time as listed in the experimental matrix. Upon completion the autoclave was rapidly cooled by the cooling water to around 98 C and then opened. The contents of autoclave, including the leach solution and remaining solids mass, were placed in a beaker and, subsequently, filtered through #40 Whatman filter paper. The solids collected in the filter paper were placed in an oven

22 22 at about 90 C and allowed to dry overnight. Once dry, the mass of the solids remaining after experimentation was determined and the final volume of the filtered solution was measured. Representative samples of the collected solids and liquids from each test were sent to Hazen Research Inc. for ICP analysis to determine the grade and recovery of molybdenum, rhenium, and copper. 6.0 Experimental Results and Discussion 6.1 Tables of Recoveries A previous report prepared by Dr. Corby Anderson analyzes acidic pressure oxidation for the recovery of molybdenum, rhenium, and copper using the same molybdenum concentrates from the Robinson Mine. The data compiled in this report was used to develop a comparison study between acidic and alkaline pressure leaching. It is important to note that based on the chalcopyrite-molybdenite Eh-pH diagrams previously mentioned molybdenum, rhenium, and copper are all soluble in acid and will subsequently be leached into the acidic solution. Table 5 contains the data for molybdenum, rhenium, and copper recovery based on the experimental factors of grind time, solids concentration, initial acidity, temperature, and time. Unlike the alkaline experiments, five factors were considered including grinding, with a total of 13 tests completed, including three replicates [1]. Grinding was not considered in the alkaline pressure oxidation testing due to time constraints but may be used in future optimization studies.

23 23 Table 6. The recoveries of molybdenum, rhenium, and copper from acidic pressure leaching, considering the experimental factors of grind time, solids concentration, initial acidity, temperature, and time [1]. As evidenced by the data, acidic pressure oxidation resulted in high levels of recovery of molybdenum, rhenium, and copper. The test that produced the highest results for molybdenum had recoveries of 84.3% molybdenum, 80.2% rhenium, and 97.9% copper. The data for alkaline pressure leaching was compiled relating the experimental factors to recovery. The solid and liquid samples from the pressure oxidation tests were examined by Hazen Research, Inc. using ICP analysis. From this, mass balances were calculated. Table 6 contains the data for the alkaline testing 4. 4 See Appendix 4: Mass balance for alkaline system.

24 24 Table 7. The recoveries of molybdenum, rhenium, and copper from alkaline pressure leaching, considering the experimental factors of time, temperature, solids concentration, and NaOH concentration. It is evident that the recoveries of molybdenum and rhenium were significantly less than in the acidic system. Further work needs to be done to improve this. The test that produced the highest values had recoveries of 48.26% molybdenum and 47.75% rhenium 5. Copper recovery was minimal from the alkaline tests as it is not soluble in an alkaline environment. 6.2 Stat-Ease Optimizations Stat-Ease Design Expert matrices and software were used for analysis and optimization for the acidic and alkaline systems. In both systems, the Stat-Ease models fit extremely well. The specific data for each may be found in Appendix 3 and 4. Figures 9, 10, & 11 contain the 5 See Appendix 5: Stat-Ease model for acidic system.

25 25 response surface models for acidic leaching of molybdenum, rhenium, and copper recovery, respectively. Additionally, the models produced the following equations for acidic pressure oxidation recoveries: Molybdenum: Logit (Mo Recovery, %) = (Grind Time) (Solids) (Initial Acidity) (Temp) (Reaction Time) (Grind Time x Solids) (Grind Time x Temp) (Solids x Temp) Rhenium: Logit (Re Recovery, %) = (Grind Time) (Solids) (Temp) (Reaction Time) (Solids x Temp) Copper: Sqrt(Cu Recovery, %) = (Grind Time) (Solids) -.008(Initial Acidity) (Temp) (Reaction Time)

26 26 Figure 9. Response surface model for molybdenum recovery from the acidic system. Figure 10. Response surface model for rhenium recovery from the acidic system.

27 27 Figure 11. Response surface model for copper recovery from the acidic system. Based on the Stat-Ease optimization modeling, good molybdenum recovery was achieved by grinding, higher temperatures, low reaction times, & low solids and most likely attributed to molybdenum solubility limits. Additionally, it was found that greater rhenium recovery was based on extended reaction times and a lower slurry solids density. Copper recovery was enhanced with higher temperatures and longer reaction times. With this said, this report found it possible to produce copper leach recoveries around 100% and rhenium and molybdenum leach recoveries around 90% within the chosen parameters [1]. Similarly, the alkaline data was interpreted with Stat-Ease 6. Figures 12, 13, & 14 contain the response surface models for alkaline leaching of molybdenum, rhenium, and copper recovery, respectively. Additionally, the models produced the following equations for alkaline pressure oxidation recoveries: 6 See Appendix 6: State-Ease model for alkaline system.

28 28 Molybdenum: Log 10 ( Mo Extraction, %) = (Time) (Solids Conc) - (1.33x10-4 )(Time x Solids Conc) Rhenium: 1/ (Re Extraction, %) = (4.49x10-4 )(Time) + (5.95x10-4 )(Solids Conc) + (8.69x10-6 )(Time x Solids) Figure 12. Response surface model for molybdenum recovery from the alkaline system.

29 29 Figure 13. Response surface model for rhenium recovery from the alkaline system. Based on the study to date, the alkaline system was not nearly as effective in extracting molybdenum and rhenium. Copper extraction was not as significant in the alkaline system, as the primary purpose of using a basic solution was to insure that copper remained insoluble and did not become part of the leach solution. Therefore, copper was not analyzed using Stat-Ease for the alkaline experiments. The models showed that molybdenum and rhenium extraction for alkaline pressure oxidation was best at long reaction times with low solids concentration. However, it is clear that the acidic system is much more efficient in molybdenum and rhenium recovery. In order to improve recoveries in the alkaline system, more experimentation and analysis is necessary. 7.0 Process Design Criteria The alkaline pressure oxidation process offers an alternative to acidic leaching of off-grade molybdenum concentrates in which rhenium has substantial credits. The benefit of this process is

30 30 the extraction and refinement of rhenium. Sending this concentrate to other facilities to extract molybdenum is costly if one could be found. With concentrates similar to those of the Robinson Mine, alkaline pressure oxidation allows selective leaching of more valuable resources such as molybdenum and rhenium from copper. With the alkaline system, the leftover copper solids can be sent to a smelter. Following this experimentation, the alkaline process still proved to be less efficient than the acidic process. At optimum temperature and time parameters the alkaline leach recovers only half that of optimum recovery of the previous study involving acidic leaching. However, with development of current practices and equipment, alkaline pressure oxidation still shows some potential for selective leaching. Moving forward with the design, it is currently recommended to use acidic pressure oxidation leaching to extract molybdenum and rhenium from the Robinson Mine concentrate. In the low-temperature pressure oxidation system considered in this study, molybdenum concentrate is added to an acidic solution and the sulfides oxidize. This is shown in the following equations. Chalcopyrite Equation: 2CuFeS O 2 + 3H 2 SO 4 2CuSO 4 + Fe 2 SO 4 + 3H 2 O + 4S o (1) The sulfur conversion to elemental form also helps lower oxygen consumption. Molybdenite Equation: MoS 2 + 3/2O 2 + H 2 SO 4 MoO 2 SO 4 + H 2 O + 2S o (2) From here, the extracted aqueous solution is further processed via liquid-solid separation for the production of copper and molybdenum.

31 31 Based upon the acidic pressure leaching results, design criteria were developed. These are presented in Figure 14. Design Criteria - Acidic Pressure Oxidation 2.1 Concentration Characteristics UNITS VALUE SOURCECOMMENTS SOURCE KEY Production tons/day 1.3 KGHM Back calculation Alibaba Mo % 34.5 CAMP Analyzed CA Dr. Corby Anderson Cu % 1.59 CAMP Analyzed CAMP Analysis Company Re % CAMP Analyzed KGHM Robinson Mine TS % CAMP Analyzed MoPOxSenior Design Team Size (P80) microns 80.4 MoPOx Analyzed MP Moisture % TBD MoPOx Tested dry SME SME Mineral Processing Handboo 2.2 Pressure Oxidation Criteria UNITS VALUE SOURCECOMMENTS USGS US Geological Survey Grind Time min 10 CAMP Through testing Work Index (Wi) kw hrs/short ton SME Estimated Percent Solids g/l 10 CAMP Through testing Reaction Time min 90 CAMP Through testing Temperature deg C 150 CAMP Through testing Pressure psig 90 CAMP Through testing Sodium Nitrite (NaNO2) (Oxidizer) g/l 2 CAMP Through testing Initial Acidity g/l 25 CAMP Through testing Final Acidity g/l 21 MoPOx Estimated calculation Stirring Speed rpm 500 CAMP Through testing Mo Recovery % 84.3 CAMP Maximum Cu Recovery % 97.9 CAMP Maximum Re Recovery % 87.9 CAMP Maximum Acid Consumption tons acid/ton 0.4 MoPOx Estimated calculation Oxygen Consumption tons O 2 /ton 0.2 MoPOx Estimated calculation Reactors # 2 MoPOx Redundant circuit Reactor Capacity tons/batch 1 MoPOx Design capacity Cycles batches/day 2 MoPOx Design capacity Recycle Leach Solution % 50 MoPOx Estimated 2.3 Liquid-Solid Separation Criteria UNITS VALUE SOURCECOMMENTS Thickener Capacity tons/day 2 MoPOx Design capacity Thickener ft 2 /tpd 12 MoPOx Estimated; confirm with testing; Filter Press ft 2 5 MoPOx calculated based on 3tpd using cost mine data 2.4 Activated Carbon Adsorption UNITS VALUE SOURCECOMMENTS Neutralization Acid Consumption tons acid/ton 1.05 MoPOx Estimated based on 50% recycle Neutralization Soda Ash Consumptiontons soda ash/ton 1.13 MoPOx Estimated based on 50% recycle Total Acid Consumption tons acid/ton 1.45 MoPOx Total POx and Neutralization acid consumptions Adsorption Capacity tpd MoPOx Calculated Adsorption Time hrs 2 MoPOx Book: Extractive Metallurgy of Molybdenum Stripping Capacity (Ammonia Effluent)L/day 437 MoPOx Calculated Stripping Time hrs 2 MoPOx Book: Extractive Metallurgy of Molybdenum 2.5 Plant Operation Criteria UNITS VALUE SOURCECOMMENTS Operators (Labor) per shift 2 MoPOx Estimated Days of Operation days/year 350 CA Standard 2.6 Economics UNITS VALUE SOURCECOMMENTS Molybdenum $/short ton MP Copper $/short ton 7300 MP Rhenium $/short ton MP Soda Ash $/short ton 135 USGS Freight on board, Wyoming Sodium Nitrite (NaNO 2 ) $/short ton 405 Alibaba Sulfuric Acid $/short ton 100 CA Oxygen $/short ton 60 CA Power $/kw hrs 0.05 CA OPEX $/short ton MoPOx Estimated calculation CAPEX $ MoPOx Estimated calculation Revenue $/short ton MoPOx Estimated calculation Figure 14. Design criteria for Robinson Mine acidic pressure oxidation system 7. 7 See Appendix 7: Design Criteria Calculations

32 32 Then a preliminary engineering design was completed for a pressure oxidation system that would successfully recover rhenium and molybdenum from a concentrate given by the Robinson Mine. The battery limits of this study are defined from the grinding of the concentrate to the carbon adsorption system. Further molybdenum and rhenium recovery from activated carbon represents existing technology as is the recovery of the minor amounts of copper solubilized (e.g. cementation, crystallization, etc.). The grinding circuit consists of a closed circuit 4 x 4 ball mill that was chosen. Two tanks, the storage tank post grinding and a pressure oxidation feed tank were designed to hold 5 tons of material and be resistant to corrosion due to the water from the wet grinding. Two pressure oxidation units were designed to create redundancy in the case that one circuit failed. The pressure oxidation unit was planned to process one ton of material, and operate twice per day. The thickener was created to treat 5 tons of material with a 6.1m diameter. A filter press with the capacity of 1 ton/hr was specified. The neutralization system was designed with a tank and mixer and uses soda ash. Finally, the carbon adsorption system was made to treat daily production of molybdenum and rhenium. 7.1 Flowsheet Shown in Figure 15 is the flowsheet created for the acidic pressure oxidation leaching of the molybdenum concentrate. The reground concentrate enters the circuit through the autoclave where it is mixed with oxygen, water, and acid. The temperature is then raised and the reaction is completed. After the autoclave process, the solids are separated from the liquids and with the valuable molybdenum, and rhenium remaining in the leach solution will be recovered using activated carbon. While it is beyond the scope of this study the copper could then be recovered by for example crystallization, cementation, or electrowinning.

33 33 Figure 15. Flowsheet for the acidic pressure oxidation leaching of the molybdenum concentrate. 7.2 Economics In order to fully evaluate the financial benefits of using pressure oxidation over other methods to treat molybdenite concentrates, a pressure oxidation system from grinding to carbon adsorption was developed. Based on the design criteria, prepared for this report, the estimated factored

34 34 capital expenditure (CAPEX) for an acidic pressure oxidation system is $7,272,437. This value includes all equipment necessary and additions for installation, piping and instrumentation, auxiliaries, and engineering costs [16]. In addition, the operating expenditure (OPEX) was calculated to be $ per ton of molybdenum concentrate treated. Amounts for the OPEX are based on the formulated design criteria and the current value of the necessary supplies. Two operators will be used to run this system. In addition, the cost for maintenance and repairs was calculated to be approximately 2% of the CAPEX [17]. Finally, revenue for the acidic system was calculated to be $12, per ton. This estimate was based on a spot price per ton of molybdenum, rhenium, and copper and the overall expected recovery 8. With these values, it was possible to determine the net present value (NPV) and the internal rate of return (IRR). The annual before-tax profit was calculated to be about $8 million. Assuming a plant life of ten years and a discount rate of 8.0%, the NPV was calculated to be approximately $46 million and the IRR was estimated to be 108.1%. The payback period for this investment would be approximately 11 months and the profitability index is Economic values are summarized in Table 7. Table 8. Summary of economics. CAPEX $7,272,437 IRR 108.1% OPEX $ per ton NPV $45,520, Revenue $12,087 per ton Payback Period months Plant Life 10 years Profitability Index 6.26 Discount Rate 8.0% Before Tax Profit $7,867,699 per year 8 See Appendix 8: Economics

35 35 A sensitivity analysis was also performed to determine which value had the greatest effect on NPV relative to the base evaluation case shown in Table 7. Based on Figure 16, it is evident that the NPV is most sensitive to the revenue per ton molybdenum concentrate treated and rhenium values dominate this. Figure 16. NPV sensitivity, showing that revenue per ton molybdenum concentrate treated has the greatest effect on NPV. 8.0 Conclusions The concentrate from the Robinson Mine contains 34.5% molybdenum, 1.59% copper, and 0.159% rhenium. The molybdenum concentration is too low for roasting, but the rhenium

36 36 content has great value. Pressure oxidation provides a new processing opportunity for such concentrates. This study was a comparison of acidic and alkaline pressure oxidation. Alkaline pressure oxidation was tested to leach molybdenum and rhenium into solution. The testing variables were time, temperature, alkaline concentration, and solid concentration. The results of the testing varied, with the best recoveries at 48.3% of the molybdenum, 45.8% of the rhenium, and only 4.32% of copper. Using Stat Ease modeling data, the significant variables for alkaline pressure leaching were found to be time and solids concentration. These results were compared to a previous study using acidic pressure leaching. Using acidic pressure oxidation gave upwards of 80% recovery in all elements. Due to increased recovery in acidic pressure leaching, the acidic process was recommended; however, with more testing, it is possible to have alkaline pressure leaching become a viable option. A preliminary design was formulated for the acidic pressure oxidation system. Using an estimated CAPEX, OPEX, and Revenue, the payback period of the acidic was approximately 11 months (assuming a plant life of 10 years and a discount rate of 8.0%). This gives a before tax profit of $7,867,699 per year with and NPV of $ about 45 M USD and an IRR of 108 %. It is suggested to move forward with the acidic pressure oxidation system due to its high recovery system and profitability potential.

37 9.0 Appendices 9.1 Appendix 1: Molybdenum Concentrate Roasting Contracts 37

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42 9.2 Appendix 2: Robinson Mine Daily Production Summary 42

43 Appendix 3: Robinson Mine Flowsheet [18]

44 Appendix 4: Mass Balance for Alkaline System Test ID Volume Hazen Liquids Results Hazen Solids Results L L g/ml g/l g/l g/l wt% wt% wt% Initial Volume Final Volume Density Cu Mo Re Cu Mo Re MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # < MP POX Test # MP POX Test # < < MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # Test ID grams Initial Solids SOLIDS grams Final Solids % Difference Solids MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test #

45 45 Test ID MOLYBDENUM grams grams grams grams grams Solid Mo Liquid Mo Final Liquid Average Calculated Mo In Mo Out Mo Out Mo Mo Extraction Solid Solid Soln In Out Extr % Extr % Mo Extr % % Head MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # Test ID RHENIUM grams grams grams grams grams Solid Re Liquid Re Final Liquid Average Calculated Re In Re Out Re Out Extraction Solid Solid Soln Re In Re Out Extr % Extr % Re Extr % % Head MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test #

46 46 Test ID COPPER grams grams grams grams grams Liquid Cu Final Liquid Average Calculated Cu In Cu Out Cu Out Extraction Solid Solid Soln Cu In Cu Out Extr % Cu Extr % % Head MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # OXYGEN Steam Oxygen Final Oxygen Oxygen Test ID Pressure In Pressure Out Consumed MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test # MP POX Test #

47 9.5 Appendix 5: Stat Ease Model for Acidic System 47

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63 9.6 Appendix 6: Stat Ease Model for Alkaline System 63

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79 Appendix 7: Design Criteria Calculations Final Acidity 25 g/l initial acidity 0.4 tons acid/ton consumed *(10g/L solids) = 4 g/l acid consumed 25 g/l 4 g/l = 21 g/l final acidity Acid and Oxygen Consumption Assume 1 ton conc Concentrate: Cu Mo 1.59% 34.50% Cu: Tons Cu Lbs Cu Grams Cu Mols Cu Mols CuFeS2 Mols O2 Grams O2 Lbs O2 Tons O2 Oxygen Tons Cu Lbs Cu Grams Cu Mols Cu Mols CuFeS2 Mols H2SO4 Grams H2SO4 Lbs H2SO4 Tons H2SO4 Acid Mo: Tons Mo Lbs Mo Grams Mo Mols Mo Mols MoS2 Mols O2 Grams O2 Lbs O2 Tons O2 Oxygen Tons Mo Lbs Mo Grams Mo Mols Mo Mols MoS2 Mols H2SO4 Grams H2SO4 Lbs H2SO4 Tons H2SO4 Acid Neutralization Acid Consumption: 1 ton of material at 10g/L solid = 2.5 tons H 2 SO tons acid consumed = 2.1 tons H 2 SO tons (50% recycle) = 1.05 tons H 2 SO 4 Soda Ash Consumption: Na2CO3 + H2SO4 Na2SO4 + CO2 + H2O Where it takes 108lb of soda ash to neutralize 98lb sulfuric acid [16] 1.08 ratio soda ash: sulfuric acid 1.05 tons acid = 1.13 tons soda ash Total Acid Consumption: 1.05 tons Neutralization Acid tons Reaction Acid = 1.45 tons acid consumed Adsorption Capacity: tons/day (2000 lb/ton) = 35.8 lbs/day Stripping Capacity (Ammonia Effluent): 12 L (20g Mo/1L)= 240g Mo=0.53 lb Mo (0.53lb/0.75L)= (309 lb/x) x =437.3 L effluent Capital Costs of Ball Mill: Using Chemical Engineering Consumer Price Index in the Chemical Engineering Magazine, the current price of a ball mill was interpreted from a linear fit from known values from 1959 to 2013 and price of a ball mill from ( )/( ) = =8.87(2103)+b b= Y(1990) = $90,000(579.4/575.4)= $

80 Appendix 8: Economics CAPEX ESTIMATE (+/- 35 %) [17], [18], [20], [21]

81 81 OPEX ESTIMATE (+/- 35 %) Revenue Estimation