Pyrometallurgical Processing Technologies for Treating High Arsenic Copper Concentrates

Transcription

1 Pyrometallurgical Processing Technologies for Treating High Arsenic Copper Concentrates Patrick R. Taylor G.S. Ansell Distinguished Professor of Chemical Metallurgy Director, Kroll Institute for Extractive Metallurgy Colorado School of Mines Enargite from Butte, Montana

2 Introduction Conventional methods to extract copper from sulfide ores using pyrometallurgy have been used for decades and more than 75% of primary copper is recovered using these methods. Even though conventional processing is highly efficient, the depletion of clean copper sulfide ore reserves requires new strategies to extract the copper with certain levels of impurities. Converter Aisle

3 Group 15 elements (As, Sb and Bi) are difficult to handle in conventional copper smelting. Environmental regulations have resulted in avoidance of high-arsenic copper sulfide concentrates and led to increased smelter penalties and lower acceptable amounts of arsenic in the concentrates treated. An example from the US is presented: Introduction Arsenic Plant

4 LIMITS & PENALTIES Example from a US Cu Smelter Element Penalty Formula Alumina $0.50 ea 0.1% > 5% Iron > 15% = increased treatment charge for more flux needed Arsenic $0.50/lb > 1% (20 lb) OR 2$/dt ea 0.1% > 0.1% Max 0.2% Barium 0.5 to 1% limit Beryllium < 10 ppm limit Bismuth ($1.10 to $7.50) /dt ea 0.1% > (0.1% to 0.4%) Max 0.4 % Cyanide < 10 ppm! Cadmium ($2.20 to $7.50) /dt ea 0.1% > (0.05% to 0.2%) Max 0.4 % Chloride BAD PLAYER, DO NOT WANT ANY Cobalt 0.5% limit Chromium $0.50 dt ea 0.1% > 3% no hex chrome, 5% max on tri v Cr Fluoride $5 dt ea 0.1% > 0.2% 0.5% max Mercury ($1.85 to $ 2) /dt ea 10 ppm > 10 ppm Magnesium Ox Normally 10% limit, desirable element in feed??? Manganese 2.0% limit Sodium 5.0% limit Nickel $2 dt ea 0.1% > 2% Phosphorus 3.0% limit Lead $1 dt ea 0.1& > 1% OR $1/lb >0.5% (more severe) Antimony BAD PLAYER, DO NOT WANT ANY Selenium 0.1% limit Tin ($1.10 to $3) dt ea 0.1% > (0.2 to 3%) Max 3 % Tellurium 0.01% limit Thallium 0.01% limit Zinc $0.50 dt ea 0.1% > 3% 4.0% limit

5 Hydrometallurgy After more than 50 years of using hydrometallurgy to recover copper from low grade ores, certain problems and issues are still a concern, especially for the treatment of high arsenic bearing ores or concentrates. Various hydrometallurgical methods have been proposed for treating high arsenic copper ores and concentrates. This is not covered in this talk. Dump Leach - Morenci

6 Arsenic? Arsenic in copper presents several problems. In the final product, cracks appear during the forming process and reduces the electrical conductivity of copper. The maximum allowable arsenic concentration in copper for electrical applications is set at 1 ppm. Copper wire product

7 Arsenic? A main objectives for the control of arsenic in mineral processing and extractive metallurgy operations is to avoid its emissions in either the gaseous and aqueous phases. This is addressed through arsenic removal and converting it to a disposable, environmentally acceptable stable compound. Sulfuric Acid Plant

8 Arsenic? There is a small market for arsenic currently. The El Indio operations is one example of this, the operations separated and recovered arsenic as the commodity As 2 O 3 until 2002, at which time operations were closed due a combination of factors, including the lack of a market for this arsenic product. Arsenic Trioxide (China)

9 USGS Estimated As 2 O 3 Production Tons

10 Process Options Both pyrometallurgical and hydrometallurgical processes have been evaluated for the extraction of copper from enargite concentrates. Most research has focused on hydrometallurgical methods, as they are generally thought to be more environmentally benign and less capital intensive. Multi-hearth Furnace

11 Pyrometallurgy High arsenic content represents a barrier to direct smelting of enargite concentrates and makes the value of the concentrates greatly diminished. For those reasons, the arsenic must be minimized before smelting to obviate the need to collect and treat volatile arsenic compounds and to decrease slag and metal contamination. Fluidized Bed - Outotec

12 Pyrometallurgy During the pyrometallurgical processing of primary and secondary materials, arsenic represents a risk and most countries require its removal from the processing residues. Controlled oxidation roasting is one of the options for pyrometallurgical pre-treatment to remove the arsenic from enargite concentrates by volatilization. The final product of the arsenic must be in a stable disposable form.

13 Pyrometallurgy Numerous studies on the use of roasting and smelting have been published for enargite concentrate treatment. Several very good review papers have been published on this topic. Several of the operating and potential processes will be discussed in this presentation.

14 Pyrometallurgy Enargite can be decomposed through roasting methods. The mechanisms for decomposition of enargite are known. Padilla (1997) showed that enargite decomposition starts at about 525 C, but maximum arsenic removal occurred between 700 C and 750 C.

15 Thermal Decomposition of Enargite Enargite starts to thermally decomposes at 550 C according to the possible reaction: 2Cu 3 AsS 4 (s) 3Cu 2 S (s) + As 2 S 3 (g) + S 2 (g) The reaction is very endothermic with an enthalpy of kj/mol.

16 Thermal Decomposition of Enargite Enargite started to decompose at about 550 C in a nitrogen atmosphere. (Cu 3 AsS 4 ) 4 (s) Cu 12 As 4 S 13 (s) + 3/2S 2 (g) Cu 12 As 4 S 13 (s) 6Cu 2 S 1+x (s) + As 4 S 4 (g) + (1.5-3x)S 2 (g) Enargite transforms to tennantite and digenite followed by the formation of chalcocite as the final decomposition product in the temperature range 575 C C. The arsenic was removed completely as arsenic sulfide species in neutral atmospheres at temperature below 727C.

17 Controlled Oxidative Roasting of Enargite In controlled oxidative atmospheres, enargite decomposes to chalcocite, sulfur dioxide and arsenic trioxide according to reaction below: 4Cu 3 AsS 4 (s) + 13O 2 (g) 6Cu 2 S (s) + 10SO 2 (g) + 2As 2 O 3 (g) Another possible reaction is (Secco et al. 1988) is: 2Cu 3 AsS 4 (s) + 2O 2 (g) 3Cu 2 S (s)+ 2SO 2 (g) + As 2 S 3 (g)

18 Controlled Oxidative Roasting of Enargite Comparison between enargite roasting in neutral and oxidative atmospheres revealed that the thermal decomposition in oxidative atmospheres is much faster than in neutral atmospheres. Temperature and oxygen potential play an important role in the rate of oxidative roasting of enargite in the temperature range 600 C - 900ºC.

19 Oxidative Roasting of Enargite The intermediate cuprous oxide in the reaction, followed by cupric oxide at extended reaction times were produced after the reaction. 4Cu 3 AsS 4 (s) + 22O 2 (g) 6Cu 2 O (s) + 16SO 2 (g) + As 4 O 6 (g) 4Cu 3 AsS 4 (s) + 25O 2 (g) 12CuO (s) + 16SO 2 (g) + As 4 O 6 (g) Enargite oxidative roasting showed a transformation to intermediate Cu 2 O and final CuO reaction product and the complete removal of arsenic in the temperature range 500 C - 900ºC.

20 Lime Roasting Lime roasting was investigated by Terry et al. (1994). The arsenic and sulfur can be captured during roasting, where As 2 O 3 and SO 2 gases react with CaO. Rapid oxidation of As 2 S 3 gas upon contacting with CaO was estimated to be the mechanism of arsenic fixation (Igiehon et al. 1994a). As 2 S 3 (g) + 3CaO (s) 1/2As 4 O 6 (g) + 3CaS (s) The reaction of As 4 O 6 gas with CaO results in the formation of calcium arsenite as showed in reaction. This reaction happened with the absence of oxygen. 3CaO (s) + 1/2As 4 O 6 (g) Ca 3 (AsO 3 ) 2 (s)

21 Lime Roasting Liu et al. (2000) performed research regarding lime roasting on a refractory gold concentrate. The SO 2 and As 2 O 3 gases were fixed based on the following reactions below: 2CaO (s) + 2SO 2 (g) + O 2 (g) 2CaSO 4 (s) 3CaO (s) + As 2 O 3 (g) + O 2 (g) Ca 3 (AsO 4 ) 2 (s) The free energy estimation for the reactions showed Ca 3 (AsO 4 ) 2 is thermodynamically more stable than CaSO 4. Based on this, theoretically, CaO will preferentially react with As 2 O 3 gas rather than SO 2. The application of hydrated lime can help in arsenic fixation.

22 Lime Roasting Taylor et. al (1991) conducted test work on lime roasting of refractory precious metal ores. This study used hydrated lime to react with arsenic and sulfur under typical roasting conditions. The results showed more than 99.9% of the arsenic and 98% of the sulfur were retained in the calcine when roasting.

23 Lime Roasting The problem with lime roasting for industrial application is that the arsenic reacted with lime stays in the solid calcine and is very difficult to remove. One possible approach would be to do this reaction in another reactor where the arsenic off gas is contacted with lime at elevated temperature. The question of the acceptability of this product for disposal would have to be addressed.

24 Soda Ash Roasting Taylor et. al (1993), studied soda ash roasting of As-Sb precious metal concentrates. The research focused on treating complex sulfide concentrate i.e. stibnite and arsenopyrite with soda ash to recover antimony and precious metals while changing arsenic and sulfur to sodium arsenate and sodium sulfate. The result showed the solid residue from roasting maintained nearly all the sulfur, arsenic, and antimony. The arsenic was removed by leaching using heated caustic solution. The arsenic can then be precipitated using a mixture of calcium hydroxide and calcium chloride or ferric salts.

25 Soda Ash Roasting Putra and Taylor evaluated soda ash roasting on two enargite concentrate samples (Putra PhD 2013) They looked at partial and complete oxidation. Conditions were found to reduce the arsenic content of the concentrate, as a water soluble compound, while maintaining much of the sulfur in the roast calcine.

26 XRD Results on Partial Oxidation Roasting 1. Fe 9 S 11, FeS 0.9, Fe 9 S 8 2. Na 2 O 2 3. Cu 2 S, CuS 2, Cu 1.96 S, CuS 4. CuO, Cu 2 1O 5. Na 2 S 6. Cu 2 As 7. Fe 8. NaAsO 2, Na 3 AsO 4 9. Na 2 SO 4 1. Cu 2 S, CuS 2, Cu 1.96 S 2. FeSO 4 3. Fe 2 O 3 4. Fe 9 S Cu 2 1O 6. F 3 O 4 7. Fe XRD pattern before water leaching XRD pattern after leaching

27 Concentrator Bin (Cu 3 AsS 4 ) Na 2 CO 3 Gas CO2 Preliminary Proposed Flowsheet Water Pelletizer Rotary Kiln Exhaust Roasted Product Bin Leached Product Leached Solution Fe 2 (SO4) 3 As Precipitation (Solid-Liquid Separation) Filtering Agitated Leaching Tanks SMELTER Thickener Filtering Leached Solid Product Solutio n Tank Solid Disposal Tank Na 2 CO 3 Bleed or EDU for Reagent Regeneration H 2 SO4

28 Soda Ash Roasting Controlled oxidation, soda ash roasting may be a method to eliminate the need for extensive arsenic collection and conversion in the off gas and acid manufacturing. The major obstacle is the cost of the soda ash and ferric sulfate for arsenic precipitation. Research is underway to evaluate reagent regeneration using EDU.

29 El Indio The enargite concentrate (23 wt.% Cu, 15 wt.% Fe, 35 wt.% S, 10 wt.% As and 0.8 wt.% Sb). Treated in a 14-hearth 6.5 m diameter Nicholas- Herreschoff roaster in the temperature range 500 C - 700ºC with an oxygen amount of less than 0.5%. This made the process essentially a neutral roasting.

30 El Indio The residence time of the feed was about 3 hours. The outlet gases were captured in two cyclones, where the collected dust was fed back to the roaster and the un-oxidized gases (sulfur and arsenic trisulfide) were oxidized in a combustion chamber with excess air through an exothermic reaction at a temperature of 750 ± 25ºC

31 El Indio Then the oxidized gases were cooled down to 350 C - 400ºC in a heat exchanger and directed to an electrostatic precipitator. The dust, which contained high precious metal values, was mixed with flotation concentrates. The dust free gases were further quenched down to 120 ± 5ºC with cold air in which a major part of the arsenic was condensed and removed from the gas phase as arsenic trioxide (white powder). The final gas stream was released to the atmosphere.

32 Sulfation Roast B. Chambers (2012) evaluated sulfation roasting of enargite Calcines produced at two temperatures, 500 and 750 C. They indicated that while high copper extraction and arsenic fixation rates could be achieved, the sulfation roasting reactions are highly exothermic and significant cooling water would need to be added. Due to these issues, it is likely that partial roasting operations would not be economically favorable in greenfield operations. However, niche applications of this process in operations with existing copper SX/EW facilities in good acid markets, have the potential to be economically favorable.

33 Sulfation Roast Flow Sheet

34 Acid Bake Process Safarzadeh, et al (2012) evaluated acid bake and leach of enargite. Preliminary experiments showed that the acid-bake leach process could be adapted and used for the treatment of enargite concentrates. Low-temperature ( ºC) sulfuric acid baking of an enargite concentrate followed by water-leaching was evaluated. Results indicate the thermal transformation of enargite to copper sulfate during sulfuric acid baking, with most of the arsenic being released from the concentrate into the gas phase for baking at 400 ºC.

35 Acid Bake Process The condition for maximum copper extraction (~100%) from the concentrate was found to be 400 ºC, sulfuric acid amount of 0.5 ml/g of concentrate, and 2 h baking time. Part of the arsenic was captured in the form of arsenopyrite (FeAsS). XRD analysis proved the presence of some arsenic in the form of claudetite (As 2 O 3 ) in the baked concentrate.

36 Acid Bake Process Possible Flow Sheet

37 CMI NESA Process Oudenne (2006) reviewed CMI NESA thirty years of the treatment of arsenical and antimonical bearing concentrates and other complex metallurgical byproducts. The importance of preliminary mineralogical characterization is emphasized as a preliminary step for the success of the roasting. Stage selective roasting appears may be an attractive and efficient route for the treatment of complex concentrates.

38 Principles Of The Selective Roasting Process It has been found that a reducing atmosphere produces the greatest arsenic removal while treating copper bearing materials containing tennantite, enargite, chalcopyrite and pyrite. In the selective roasting process, the complex sulfide materials are heated in reducing or starved air conditions. Depending on the specific characteristics of the concentrates, such as particle size, different controllable variables appear to influence the volatilization of Cd, As, Sb, Bi and Hg during roasting. - Ultimate temperature - Heating rate - Gaseous atmosphere - Presence of sulfur

39 Selective Roasting Process Selective roasting is carried out under the conditions explain above to facilitate impurities removal i.e. adequate ultimate temperature, low and smooth heating rate and reducing atmosphere with optimized CO/CO 2 ratio. A roasting furnace operated under a single given atmosphere such as in a fluidized bed roaster, makes it difficult to obtain appropriate roasting conditions for complex sulfide materials.

40 Principles Of The Selective Roasting Process Since the volatile sulfides are unaffected by a reducing atmosphere, it is thus possible to effect an elimination of them from non volatile minerals. Moreover, in a multiple stage reactor, the separate control of temperature and atmosphere is possible for each hearth of the reactor and equally important above and within the material bed.

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42 Principles Of The Selective Roasting Process

43 Example Of Selective Roasting Enargite selective roasting. The purpose of the pretreatment by selective roasting of this enargite concentrate is the removal of a maximum of the contained arsenic and antimony to obtain a saleable product, while retaining a high sulfur level for smelting. The copper flotation concentrate contains enargite (Cu 3 AsS 4 ), covellite (CuS), chalcopyrite (CuFeS 2 ), pyrite (FeS 2 ), elemental sulfur and quartz (SiO 2 ) as major minerals. The average size of the flotation concentrate is in the range of 40 microns.

44 Example Of Selective Roasting In a reducing atmosphere, the main sulfides minerals will decompose according to the following reactions : 2Cu 3 AsS 4 = Cu 2 S+4CuS+As 2 S 3 (g) 2FeS 2 = 2FeS+S 2 (g) 8FeAsS = 4FeAs+4FeS+As 4 S 4 (g) These decompositions proceed as the concentrate is progressively heated in a controlled atmosphere. Moreover as the elimination of volatile sulfides occurs, one observes that the sintering temperature of the material being roasted is increasing.

45 Example Of Selective Roasting The oxygen-free atmosphere in and just above the reacting bed is protected because oxygen is consumed by reacting with rising volatile sulfides and labile sulfur arising from the lower hearths. Part of the heat used in the multistage reactor is generated by controlling air flow into the upper hearths taking advantage of the exothermic reactions with the rising gases : S 2 (g) + 2O 2 (g) = 2 SO 2 (g) As 4 S 4 (g)+7o 2 (g) = As 2 O 3 (g)+4so 2 (g) In the bottom hearths, external fuel is burnt in order to control hearth temperatures and gas atmosphere. The presence of pyrite in the concentrate is favorable because it helps the volatilization of the As as arsenic sulfide.

46 Example Of Selective Roasting

47 Ministro Hales Codelco and Outotec (very little public information) The treatment of Ministro Hales deposit in Chile by Codelco (McElroy et al. 2008; Mayhew et al. 2011), which will start in 2013, expects a yearly throughput of 550,000 tonnes of copper concentrate and 250,000 tonnes of sulfuric acid production. Controlled oxidation (or neutral roast) to remove arsenic as a vapor compound. Arsenic dust is collected and converted to disposable form or stockpiled. Outotec in Frankfurt, Germany is operating a 25 kg per hour pilot plant that uses the roasting method to treat copper concentrate (Ondrey, 2012).

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49 Smelters The ASARCO smelter at El Paso, Texas gives a maximum of 0.2% As in the copper concentrate that can be the feed (ASARCO El Paso Smelting Processes, 2010). Enargite concentrates are smelted directly by a few copper smelters but most smelters limit their arsenic input for environmental reasons and only process clean copper concentrates, containing less than about 0.5% As (Peacey et al., 2010).

50 Smelters The Lepanto roaster at Lepanto Consolidated Mining Company, Philippines treated about 180 tpd of enargite concentrates containing 11% As, 31% Cu, 15% Fe, 34% S and produced calcine containing 0.3% As, 43% Cu, 23% Fe, and 20% S. The Boliden roaster at Boliden Oy, Finland, with a capacity of up to 800 tpd concentrate, treated concentrates containing 1-2% As and typically produced a calcine containing about 0.1% As ( Peacey et al., 2010).

51 Smelters Dundee Precious Metals Inc. from Chelopec, Bulgaria transports the enargite concentrates (175,000 t/a) to the NCS smelter in Tsumeb, Namibia. This smelter can treat high arsenic and lead bearing copper concentrate. Not many smelters in the world can do that. Copper blister and arsenic trioxide (As 2 O 3 ) were produced from this smelter (Dundee Precious Metals Inc. 2012a).

52 Arsenic issues and arsenic precipitation Several issues result from the presence of arsenic in arsenic-bearing concentrates when entering the processing circuit: Increased operating costs Complicating metal extraction circuits Diminishing the final product purity Possible health and environmental concerns Safe disposal problems.

53 Arsenic Collection As 4 O 6 or As 2 S 3 vapors is the gas phase may be present in some streams when extracting the copper. Electrostatic precipitators or wet gas scrubbers, are used to collect As 2 O 3 dusts. As 2 O 3 dust cannot be stockpiled because of the long term problems associated with the high solubility in water and toxicity.

54 Example Smelter Arsenic Treatment

55 Smelter Arsenic The Horne smelter in Rouyn-Noranda, Quebec has developed its own process for the precipitation of arsenic in its effluent treatment plant. The process is using the AMD (acid mine drainage) as the agent and produce arsenical ferrihydrite. The process is increasing the ph to about 4-5 and oxygen injected to oxidize ferrous to ferric ion, the second stage reaction tank, the ph is increased to about 9 to precipitate arsenic to 0.5 ppm (from 2500 ppm). This is a well proven process in the field, and the product is stored in a monitored pond.

56 Smelter Arsenic Inco s Copper Cliff Smelter in Sudbury installed a weak acid treatment process that also precipitates arsenic as arsenical ferrihydrites using SO 2 and H 2 SO 4 respectively, the process has been conducted for 30 years. The Saganoseki copper smelter in Japan precipitates the arsenic in solution, from its converter dust leach and its acid plant weak acid bleed, as arsenic sulfide using sodium hydrosulfide. Codelco has conducted bench scale tests using pyrite and low grade copper concentrates and pilot plant runs were carried out at SGS Lakefield treating a weak acid containing 14.8 g/l As and 26 g/l H 2 SO 4. The arsenic precipitated out of solution quickly down to about 1.5 g/l As. Stable material produced and stored in a concrete pit on site (Peacey et al., 2010).

57 Arsenic removal from Solution Various methods have been studied for removing arsenic from solution: chemical coagulation precipitation (Pal et al., 2007) electrolysis (Emamjomeh and Sivakumar, 2009) liquid-liquid extraction (Guell et al., 2010) ion exchange (Anirudhan and Jalajamony, 2010 reverse osmosis (Walker et al., 2008) membrane separation (Fagarassy et al., 2009) adsorption (Yenial, 2012) flotation (Aydoğan, 2011).

58 Ferrihydrate Precipitation Co-precipitation removal of arsenic by ferrihydrite precipitation from solutions containing both dissolved iron and arsenic species. Post-precipitation is removal of arsenic from solutions by the addition of a solution of aqueous ferric bearing species at a ph where ferrihydrite forms (usually at ph 7 to 8) adsorption is removal of arsenic from solutions by exposing arsenic bearing solutions to a previously precipitated ferrihydrite solid. The precipitation depends on time, temperature, ph, Fe/As mole ratio, agitation rate, arsenic valence state, associated ion, etc (Patnaik, 2004).

59 Ferric Precipitation The precipitation of dissolved arsenic with ferric ions has been declared by The United States Environmental Protection Agency (EPA) as The Best Demonstrated Available Technology (BDAT) (Riveros et al. 2001; Rosengrant and Fargo 1990). Several factors that have influences on the stability of any disposed arsenic compound: characteristics of the disposal location, crystallinity and size distribution of the compound, and the presence of oxygen or any complexing agents.

60 Ferric Precipitation Arsenic ferrihydrite compounds is formed at low temperatures and atmospheric pressure and it is amorphous and voluminous. According to the EPA, arsenic ferrihydrite compounds are acceptable for practical arsenic disposal operations, releasing less arsenic than the mandatory requirements in many countries. Scorodite was formed at high temperature and pressure conditions. It produces a low-volume, dense, crystalline precipitate that has lower solubility (less than 1 mg/l As at ph 5) than arsenical ferrihydrite. However, recent research shows the possibility of scorodite formation under atmospheric pressure conditions; but the industrial application of this process has not yet been reported.

61 Factors in Processing Some general difficulties of applying certain arsenic treatment method are: A wide range of arsenic concentrations Effects of other elements and their variable concentrations in the solution The need to adjust ph for optimal removal Optimized dose Proper operation and maintenance Safe disposal of solid arsenic waste

62 Conclusions Smelters generally accept copper concentrates with low arsenic levels (less than 0.2% to 0.5%) due to environmental and product purity concerns. Treating the waste stream from high arsenic copper concentrates requires significant investment. The separation and collection of arsenic as arsenic trioxide is problematic, primarily due to its toxicity and lack of market value.

63 Conclusions Pyrometallurgical processing has several concerns: - the strict environmental regulations on arsenic release - the complexity of gas/dust capture - separation facilities - and the stabilization of the final arsenic compound Recent application in the industry to treat high arsenic concentrate (enargite) were Barrick's El Indio smelter in Chile and Xstrata's Home smelter in Canada. Both employed reductive roasting-smelting process for the treatment of enargite concentrates.

64 Conclusions Despite the perceived shortcomings for roasting to selectively volatilize arsenic, the process could an option under specific conditions, provided that a arsenic precipitation process is possible. Roasting is the only process that has been operated commercially for enargite concentrates; therefore the level of project risk is lower, i.e. based on past commercial experience. Another advantage of roasting is it is less sensitive to feed mineralogy than hydrometallurgical options and arsenic disposal as a compact crystalline compound with low solubility would appear possible.

65 Thank you!!! It has been a pleasure to be at PERUMIN again.