Energy Efficient and Low CO2 Emission ...

Energy Efficient and Low CO2 Emission ‘THIOMETALLURGY’ Neale R. Neelameggham, Robert E. Brown and Brian R. Davis [Preprint March 21, 2014 – of paper published in JOM September 2014]

Abstract Extractive Metallurgy has utilized free or combined sulfur as both the raw material and energy material in carrying out economical manufacture of several metals in millions of tons per year quantities over the past century. This has controlled carbon emissions in an unintentional fashion and out of necessity as the ores in many cases have been sulfides to start with; and the benefits of heat generation by the sulfides reacting with oxygen in the process steps –have avoided the use of carbon as a fuel in providing the reaction temperatures. In this paper we will show the inherent benefits of ‘Thio-metallurgy’ which uses sulfur in the extraction of metals in alleviating CO2 emissions, as well as its ability to provide a cost effective energy material solution. Such solutions are not only applicable to existing base metal production and as the authors will show are applicable to newer processes in the production of other metals and chemicals –such as alkaline earth metals, titanium, and to an extent aluminum in an indirect fashion. Introduction The word sulfur has the Sanskrit origin ‘sulvere’ adapted into Latin as ‘sulfur’. Greeks called it ‘thio’ and many of the radicals and compounds of sulfur are referred as thio-compounds. Sulfur, even though it is in group VI of the periodic table below oxygen [the universal oxidant in air], is a reducing agent like carbon instead of being an oxidant at its elemental state. This happens mainly from its multi-valence and the affinity for elemental sulfur to oxygen. Sulfur metabolizing bacteria are referred to as thiobacillus, etc. Ramanathan, Scripps Institute of Oceanography, who in 1975 noted the highest global warming potential from fluocarbons [ 1], later started modeling the benefits of global cooling by radiative forcing by aerosols from natural events such as volcanic eruptions as well as anthropogenic aerosols. The results of the Indian Ocean [INDOEX] experiments in the late 1990’s were summarized by Vogelman, Ramanathan andS.K.Satheesh [2] as ‘… A persistent haze layer that spread over most of the northern Indian Ocean during wintertime was discovered. The optical thickness of this layer ranges from as high as 0.5 to 0.6 in the northern Arabian Sea and Bay of Bengal to about 0.1 to 0.2 in the southern equatorial Indian Ocean. The layer, a complex mix of organics, black carbon, sulfates, nitrates, and other species, subjects the lower atmosphere to a strong radiative heating and a larger reduction in the solar heating of the ocean.’ Further studies were carried out by Ramanathan, Paul Crutzen and others in the early 2000s. This is the same observation earlier of smog from smoke stacks and dilute sulfur oxides during the 60’s which lead to the clean-up of environmental pollution. In 2006, Paul J. Crutzen noted in his essay “Albedo Enhancement By Stratospheric Sulfur Injections: A Contribution To Resolve A Policy Dilemma’ that he has data to support his claim that his calculations using the best models available have shown that injecting 1 million tonnes of sulfur a year would cool down the climate so the greenhouse effect is wiped out.

Apparently the theory uses the idea that an added layer of sulfates in the stratosphere, approximately 16 kilometres above the earth, would reflect sunlight back into space and reduce the amount of solar radiation reaching the Earth's surface. [3]. Most of us must have heard news items that Bill Gates, Microsoft founder has been funding some of this research since 2010. In spite of the use of sulfur dispersion in the stratosphere, these suggested solutions continue in the similar line as ‘solution to pollution is dilution’ – as in the 1970s by dispersing the pollution by increasing the height of stack [here they want to go to stratosphere – some 50000 feet above earth’s surface, in this case to cause unclean air to allow energy inefficiencies to continue at the surface]. The authors want to note that it is more prudent to handle the ‘solution by concentration at the source’ as extractive metallurgists of base metals have done it by improving process efficiency, and energy efficiency by capturing at the source in a concentrated form and minimizing waste. Metal Extraction with Minimal Carbon Use Little did any of these discussions recently of global warming abatement, give the credit to the best practices of metal extraction, especially the base metals. These metals are produced in millions of tons per year and have been done with low amounts of carbon dioxide emissions. How are these done? Copper is extracted from sulfides, lead and zinc are extracted from sulfides, nickel and cobalt, molybdenum is extracted from sulfides. The fuel value of the sulfides is effectively utilized in this metal extraction. The scientists working on this for several decades developed efficient techniques – to see the benefits of flash smelting technologies in cutting down the costs of operation as well as cutting down the pollution resulting from release of dilute gaseous emissions – which could have aided in the radiative forcing technique mentioned earlier. Valence States of Sulfur According to valence theories, sulfur exhibits several valence states from -2, -1, 0, +4, and +6. Negative divalency is seen in sulfides in compounds such as H 2S, Na2S, FeS, PbS, ZnS, etc. Negative mono-valency is seen in compounds such as sodium disulfide [Na 2S2], and FeS2. Zero valence state is the naturally occurring elemental sulfur. In this state it occurs in orthorhombic as well as in monoclinic forms. It is denoted as S 8 to show the polymeric nature of the element. The +4 valence state is seen in compounds such as sulfur dioxide, and sulfites; while the +6 valence state is seen in sulfur trioxide, sulfuric acid and sulfates. Interestingly, the compound ion thiosulfate, which is formed by addition of sulfur to sulfite ion, S2O3-2 is not a +2 valence sulfur, but it has two sulfur atoms which are not equivalent. This is a sulfate ion SO4-2 in which one other oxygen atom is replaced by a sulfur atom– the central sulfur atom may be assigned oxidation number +6, and the attached sulfur atom oxidation number -2 [similar to the other three oxygen atoms]. [4] Thiosulfate ion is easily oxidized to tetrathionate ion, S4O6-2 which is a complex ion. Yet another complex oxo-ion of sulfur is hexathionate, S6O6-2. The other Group VI elements selenium and tellurium behave similar to sulfur in their multivalence properties.

Sulfur as Fuel The negative di or mono-valent sulfur can interact with oxygen atoms in further getting oxidized. Such a property makes the sulfides of metals to have the fuel like property of carbon. The sulfides get oxidized to sulfur dioxide and further to sulfur trioxide this property is also seen with elemental sulfur. . Heats of combustion of sulfur are comparable to carbon. S + O2  SO2 DH = -296.4 kJ/mole ---[A] S + 1.5 O2  SO3 DH = -397.8 kJ/mole ---[B] C + 0.5 O2  CO DH = -104.4 kJ/mole ---[C] C + O2  CO2 DH = -384.1 kJ/mole ---[D] Elemental sulfur can become sulfur oxides – similar to carbon which can become carbon oxides. Unlike carbon oxides which linger in the atmosphere without condensing at ambient and stratospheric conditions, sulfur oxides get dissipated easily by reaction with moisture in the atmosphere and thus do not contribute to the global warming potential. Sulfur oxides become sulfurous and sulfuric acid and forming the acid mist dissipation – alluded to by Paul Crutzen earlier. One should note that water vapor is a greenhouse gas and it has to give its heat [global warming] to nearest air molecule before becoming a cloud in minimizing further warming from the sun. Increased water vapor in the atmosphere thus means further global warming as one of the recent reports by Willett, et al showed that the rise in humidity caused by people, in the air near Earth's surface, rose 2.2 percent in less than three decades since 1973 – confirming that it is difficult to convert water vapor into clouds without heating something else during condensation – thus continuing global warming [5]. In this sense, hydrogen falls in the category of global warming reductant like carbon, unlike sulfur…We should, however, not discount the global warming from the inefficient energy usage in processes – which may happen with sulfur as well by inadequate design. Neelameggham has suggested how to handle the combined effort of CO 2 and water vapor in his –soda-fuel metallurgy discussions. [6] Copper Flash Smelting The technology of metal sulfide reduction has developed to be a clean air technology during the past six decades. These technologies were developed not to abate CO 2 – but to minimize the cost of production. Cost minimization came from minimizing waste, operating with little dilution and reduced pollution by capture at the source. The economic benefits of operations with minimal dilution, forming concentrated sulfur oxides which are then captured and marketed as sulfuric acid have long been a norm in base metal industry. The following reactions summarizes flash smelting of copper [Flash smelting] CuFeS2  Cu2S + FeS ---[E] FeS + O2  FexOy + SO2 ---[F] FexOy +Z SiO2  FexOy.. Z SiO2 [slag] ---[G] Cu2S + O2 = 2 Cu[matte] + SO2 ---[H] [Flash Converting]

Cu[matte] + S[matte] + O2  Cu [blister] + SO2 Cu [blister] (electrorefining)  Cu [cathodes]

---[I] ---[J]

Almost a decade before Kyoto protocol on minimizing global warming, J.A. Asteljoki, et al, noted that ‘The size of the gas handling equipment is also reduced, and the continuous flow of high strength SO2 gas allows optimization of acid plant design. In a recent study for a smelter treating 500,000 mtpy of chalcopyrite concentrates, the gas cleaning plant would have to treat 60,000 SCFM (100,000 NM3/hr.) of process gases from a flash furnace and a Peirce-Smith converter. Using flash converting, the gas cleaning requirement is reduced to 25,000 SCFM (43,000 NM3/hr.). Preliminary estimates indicate potential savings of up to 20% in both capital and operating costs for a "greenfield" smelting complex. [7] Gold – ThioHydro metallurgical Process Thiosulfate ions are known to form complex ions, with noble metal gold such as Au(S2O3)2 3- . This property is used for cyanide free extraction of gold from refractory ores, followed by anion exchange removal of gold thiosulfate complexes from leach liquor. ‘Cyanide free process’ also means low carbon process. R.Y.Wan, J.D.Miller, and J.Li called this a ‘Thio hydrometallurgical’ process in 2005. [8] This paper discusses use of other sulfur complexes such as thio-urea Au(SC(NH2)2)2+, and Thiocyanate Au(SCN)2- and compares with conventional cyanide complexation reactions Zinc Sulfide to Zinc The aqueous electrolysis process, Roast-Leach-Electrowin process, is more widely used than the pyro metallurgical processes where carbon is used to remove the oxygen from ZnO. The electrolysis process consists of 4 steps: leaching, purification, electrolysis, and melting and casting. The following reactions take place in the sphalerite [ ZnS] roasting followed by zinc sulfate formation which is electrolyzed making zinc metal with little or no carbon as a reducing agent.. The carbon dioxide penalty may occur if the electricity is generated using carbonaceous fuels, instead of using hydro-power or other non-carbon alternate energy. 2 ZnS + 3 O2  2 ZnO + 2 SO2 2 SO2 + O2 + 2 H2O  2 H2SO4 ZnO + H2SO4 + H2O  ZnSO4[aq] + H2O

---[K] ---[L] ---[M]

Nickel Sulfide Smelting In the case of nickel smelting A.E.Warner, et al noted that ‘the output of flash smelters accounts for nearly 70% of the primary metal produced from nickel sulfide sources ... Electric - furnace smelters produce the balance. The key merits of flash smelting are very low electrical and fossil fuel energy consumption and generation of a continuous, low-volume, SO 2-rich process gas stream amenable to processing in an acid plant.’ [9] Lead Sulfide Hydrometallurgical Process

Agnes Y. Lee, et al reported on the U.S. Bureau of Mines research on hydrometallurgical conversion of lead sulfide to lead and elemental sulfur. The steps were (a) leaching with H 202 , Pb02 , and recycled fluosilicic acid at 95° C to produce a solution of PbSiF 6 and a residue containing elemental S, (b ) electrowinning of the PbSiF6 solution at 35° C to produce 99.99 pct Pb metal and H2 SiF6 , and (c) Sulfur recovery by solvent extraction, leaving a residue containing Cu, Ag, and other metal values. [10 ,11 ].

Thio metallurgy for Green Alkaline Earth Metals Neelameggham and Brown have envisioned proprietary Thiometallurgical processes which can minimize carbon dioxide emissions for the following metals and compounds.[12] It is shown that Green Alkaline Earth Metals and chemicals can be made from naturally occurring sulfates of magnesium and calcium. Naturally occurring sulfates of magnesium include epsomite [MgSO4.7H2O], kieserite [MgSO4.H2O], as well as mixed sulfates with potassium – schoenite, langbeinite, etc. We all know gypsum, the naturally occurring sulfate of calcium CaSO4.2H2O and anhydrite CaSO4. Use of sulfates as a raw material overcomes the CO2 penalties arising from the use of alkaline earth carbonates such as calcite (CaCO 3), magnesite (MgCO3) or dolomite (CaCO3.MgCO3). In the case of using naturally available magnesium chloride brines calcium carbonate is used in making the calcium chloride used in removing the sulfates in the brine – thus causing CO2 emissions. In addition, purified magnesium chloride brine is evaporated using fossil fuels in making anhydrous magnesium chloride –which creates additional carbon dioxide emissions. MgCl2[aq] + H2O [aq]  MgCl2 + zH2O[crystal] + y MgO + y HCl +H2O[v] (~300 oC) ---[M] The hydrolysis of magnesium chloride creates residual H 2O [crystal] and MgO [crystal] – this can be removed by dehydration with additional HCl vapors [~400 oC] or by molten salt chlorination [750 – 850 oC]. Processes such as the Norsk Hydro Canada [now closed], and similar processes used magnesium carbonate as the raw material in making the magnesium chloride feed precursors, with associated carbon dioxide emissions. Many of the newer processes using magnesium oxide as raw material avoid the ‘inconvenient truth’ regarding the carbon emissions associated with the present day methods of preparing the pure magnesium oxide feed and do not announce the values in their publications. In 1808, Davy and in 1884 – Gerhard and Smith, have been noted to have used magnesium sulfate aqueous solution as the raw material in trying to isolate elemental magnesium, without much success.[13] There have been a few patents in the 1930 -40 period of using carbonaceous fuel to convert magnesium sulfate into magnesium oxides, sulfur oxides and carbon oxides. After that, magnesium sulfate use has not been used in providing the pure magnesium oxide with near zero carbon emissions – as shown below. In many cases millions of tons of magnesium sulfate is wasted in evaporite basins without realizing their value. Such

evaporite basins include ponds at the banks of Great Salt Lake, other bittern effluent ponds worldwide, Aral sea dried basin part of Kazakhstan – Uzbekistan area. The following reactions show the decomposition temperatures of these anhydrous sulfates of alkaline earth metals by direct heat which can be reduced considerably using sulfur as the reductant. MgSO4.7 H2O [epsomite] MgSO4.H2O (~ 80o C) ---[N] o MgSO4.H2O [kieserite]  MgSO4 (~ 250 C) ---[O] MgSO4 == MgO + SO2 + 0.5 O2 (~ 1000 oC) ---[P] MgSO4 + 0.5 S == MgO + 1.5 SO2 ( ~ 625 oC) ---[Q] CaSO4 == CaO + SO2 + 0.5 O2 ( ~1600 oC) ---[R] CaSO4 + 0.5 S == CaO + 1.5 SO2 (~ 1200 oC) ---[S] MgSO3 == MgO + SO2 ---[T] CaSO3 == CaO + SO2 ---[U] It is shown that magnesium sulfate anhydrous is easier to prepare than magnesium chloride anhydrous at about 250 oC, while MgSO4.H2O can be made at a low temperature of about 80 oC unlike MgCl2.H2O which requires a temperature of about 250 oC or higher and associated with hydrolysis. Making kieserite at 80 oC is amenable to solar energy applications, avoiding carbonaceous fuel use. It is further noted that the ‘green’ MgO or anhydrous MgSO 4 can be converted to anhydrous magnesium chloride by Ind LLC’s process for conventional electrolysis, with a lower energy cost. Such a process is amenable for ‘brownfield’ conversions in existing magnesium production facilities. The ‘green’ MgO can be co-produced with green ‘CaO’ using gypsum or sulfite for use in processes such as silico-thermal process which claims hydro-electric powered Ferro silicon reduction production of 2.2 kg CO2/kg Mg. Such a ‘carbon credit’ bearing process can be applied by present producers in their ‘brown’ field expansions. The ‘green’ MgO or ‘CaO’ process is suitable for regenerating the oxides from sulfites resulting in flue gas desulfurizing scrubbers. Other synergies include production of ‘fully or partially green’ Portland cement using calcium oxide generated from gypsum with clay to form the cement clinker – the endothermic heat being supplied by sulfur for fully green cement and carbonaceous fuel supplying the heat in the partly green cement. In the 1960’s U.K. atomic energy used calcium sulfide as the feed to calcium metal electrolyzer – this is another early application of thiometallurgy for alkaline earth metals. Sulfur removal from steel has been done mainly using either calcium carbide or magnesium during the last 30 years – showing the interaction between alkaline earth metals and sulfur in metallurgy.

Thio metallurgy for Titanium and titanium chemicals with low carbon foot-print Essentially carbon free titanium dioxide has been made for over a century using sulfuric acid digestion of ilmenite, and other iron bearing titanium oxides, by the sulfate process. In the year 2000, J.Miller and B.R.Davis disclosed an improved sulfuric acid digestion practice using a liquid phase approach. [14] Lumsden discussed reactions of sulfuryl chloride in making ferrous chloride along with titanium tetrachloride [15]. Hill described suspending ores such as TiO 2 – rutile, in molten sulfur [excess sulfur] and chlorinating the ore to make volatile chlorides and sulfur dioxide. The reactions are carried out using ferric chloride, sulfuryl chloride or chlorine and the temperatures being in the range 250 to 350 oC while keeping sulfur in the molten liquid state. [16]. These processes also fall under ‘Thiometallurgical’ conversion of iron bearing titanium oxides. Neelameggham and Davis is further developing a proprietary green titania process while trying to avoid the formation of waste copperas – FeSO4.7H2O – while producing marketable sulfuric acid from the sulfur oxides which are formed. [17] They consider reactions of sequential partial oxidation of iron as a water soluble ferrous chloride or sulfate if a market for that exists for water treatment. These processes use sulfur instead of sulfuric acid in controlling the products which are formed. Some of the reactions used are FeTiO3 + Cl2 + 0.5 S  FeCl2 + 0.5 SO2 + TiO2 FeTiO3 + 2.5 O2 + 2 S  FeSO4+ SO2 + TiO2 FeTiO3 + S + O2  FeO + SO2 + TiO2

---[V] ---[W] ---[X]

Other variations of these reactions are possible including the use of controlled amounts of water. Figure 1 show thermodynamic equilibria components at low temperatures such as will occur in wet grinding reactions of ilmenite with sulfur along with oxygen additions. These reactions are easily carried out in some of the equipment that have been used by the sulfate process of making TiO2 – such as in rod mills or by other mechano chemical approaches using high energy milling, followed by leaching and separation steps. The need for partial chlorination may become essential in removing other impurities in the ore such as silica, vanadium, etc. There have been patents of using sulfur chlorination of ilmenites in the 1930 – 1980 periods; those processes also fall under ‘thiometallurgical’ conversion of iron bearing titanites. Details of ferrous and ferric state in the ore and methods to handle them are not shown in this general paper on ‘thiometallurgical’ approach to making titanium compounds.

Thio metallurgy in Aluminum Production In the case of aluminum production, use of aluminum sulfide as starting material instead of aluminum oxide has been known to reduce energy consumption in the process. Nguyen Q Minh, et al patented a process to make metallic aluminum by the electrolysis of A1 2S3 at 700°800° C. in a chloride melt composed of one or more alkali metal chlorides, and one or more alkaline earth metal chlorides and/or aluminum chloride. [18] This provided improved operating characteristics by lowering the cell voltage compared to typical Hall Heroult Process, thus lowering energy costs by the combination of operations at a lower temperature and lower cell voltage. Anode produced sulfur vapors without attacking the graphite electrode unlike in the Hall Heroult cell the oxygen release at carbon anodes resulting in carbon oxides. The authors, however, omits the method by which aluminum sulfide is formed as the cell feed. By-product Credit Improves Metal Production Economics When one looks at the details of reagent costs the cost of sulfur is competitive with that of carbon, especially since more sulfur removal from diesel fuel and other transportation fuels

have been mandated – petroleum refineries have become suppliers of sulfur, besides the natural gas wells supplying sulfur. When sulfur oxides are released in a concentrated form they are amenable to be turned into marketable sulfuric acid as done in smelters. This by-product credit will pay for the sulfur used as a reducing agent or as an energy matter. Such a by-product credit is not available when using carbon as a fuel or a reducing agent. Power plants are still slow in reducing their costs while abating pollution. Thus Thio-metallurgy provides unique cost effective carbon abatement strategies. References 1. V. Ramanathan, ‘ Atmospheric Fluocarbons – A large increase in Global climate’, Environmental Conservation, Volume 3, Issue 02 -Summer 1976, p 90 2. A. M. Vogelmann, V. Ramanathan, and S. K. Satheesh, ‘Aerosol Forcing from the Indian Ocean Experiment and the ARM-SGP’, Tenth ARM Science Team Meeting Proceedings, San Antonio, Texas, March 13-17, 2000 3. P. Crutzen, ‘Albedo Enhancement By Stratospheric Sulfur Injections: A Contribution To Resolve A Policy Dilemma?’, Climatic Change (2006) 77: 211–219. 4. Linus Pauling, General Chemistry, W.H.Freeman and Company, 1947, 312 5. Katharine M Willett, Philip D Jones, Peter W Thorne and Nathan P Gillett, ‘A comparison of large scale changes in surface humidity over land in observations and CMIP3 general circulation models’, 2010 Environ. Res. Lett. Volume 5 Number 2 6. R. Neelameggham, ‘Soda Fuel Cycle Metallurgy –Choices For CO2 Reduction’ - Proceedings of CO2 Reduction Metallurgy Symposium, Ed. Neale R Neelameggham & Ramana G Reddy,,TMS Annual Meeting 2008, New Orleans, p.135-146. 7. J. A. Asteljoki, L. K. Bailey, D. B. George and D. W. Rodolff, Flash Converting - Continuous Converting of Copper Mattes, JOURNAL OF METALS· May 1985, 21-24 8. R.Y. Wan, J.D. Miller and J. Li, ‘ Thiohydrometallurgical Processes for Gold Recovery’, Innovations in Natural Resource Processing - Proceedings of the Jan. D. Miller Symposium, edited by Courtney Young, Jon J. Kellar, Michael L. Free, J. Drelich and R.P.King, Society of Mining Engineers, 2005, 223-244 9. A.E.M. Warner, C.M. Díaz, A.D. Dalvi, P.J. Mackey, A.V. Tarasov, and R.T. Jones, ‘ JOM World Nonferrous Smelter Survey Part IV: Nickel: Sulfide’, JOM, April 2007, 58-72 10. Agnes Y. Lee, Ann M. Wethington, and Ernest R. Cole, Jr., ‘Hydrometallurgical Process for Producing Lead and Elemental Sulfur From Galena Concentrates’, Report of Investigations 9055 11. Agnes Y. Lee, Ann M. Wethington, and Ernest R. Cole, Jr., ‘Pressure Leaching of Galena Concentrates To Recover Lead Metal and Elemental Sulfur’, Report of Investigations 9314 12. Neelameggham and Brown, “Alkaline earth oxides for green processes for metal and other material”. U.S. Patent application April 2013 13. J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol IV, Sulfur and Selenium, Longmans and Green, 1929, 254 14. Miller.J., Davis, B.R., Rahm,J.A., Madsen, E.J., ‘Continuous non-polluting liquid phase titanium dioxide process and apparatus’, U.S. 6,048,505 April 11, 2000 . 15. Lumsden, J., ‘Chlorination of iron-containing materials, United States Patent 4179489, Dec. 18, 1979 16. Hill.C.T., ‘Chlorination of metal oxides’, United States Patent 2970887, February 7,1961 .

17. Neelameggham and Davis, “Production of titanium compounds and metal by sustainable methods”. U.S. Patent application April 2013 18. Nguyen Q. Minh, Raouf O. Loutfy, and Neng-Ping Yao, ‘Production of aluminum metal by electrolysis of aluminum sulfide’, U.S.Patent 4464234, August 7, 1984