Abernethy, F.

Anand, M.

Barber, S. J.

Morse, A.

Sargeant, Hannah

Sheridan, S.

Wright, I. P.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsHydrogen reduction of ilmenite as an ISRUdemonstration for ProSPAConference or Workshop ItemHow to cite:Sargeant, H.; Abernethy, F.; Wright, I. P.; Barber, S.; Sheridan, S.; Morse, A. and Anand, M. (2017). Hydrogenreduction of ilmenite as an ISRU demonstration for ProSPA. In: European Lunar Symposium (ELS) 2017, 2-3 May2017, Munster, Germany.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukHydrogen reduction of ilmenite as an ISRU demonstration for ProSPA.  H. Sargeant1, F. Abernethy1, I. Wright1, S. Barber1, S. Sheridan1, A. Morse1, and M. Anand1. 1 School of Physical Sciences, The Open Universi-ty, Walton Hall, Milton Keynes, UK. E-mail: Hannah.sargeant@open.ac.uk   Introduction:  The next step for human explora-tion in space will begin with a return to the Moon, where In-Situ Resource Utilisation (ISRU) will be crucial in supporting an extended human presence on the lunar surface that is less dependent on the Earth’s resources. To determine the feasibility of lunar ISRU we must first understand the resources available. A number of missions are planned for the coming dec-ade including the Luna-27 surface lander, which will provide a better understanding of the ISRU capabili-ties of the lunar south pole [1]. On board the Luna-27 mission will be ESA’s Package for Resource Observation and in-situ Pro-specting for Exploration, Commercial exploitation and Transportation (PROSPECT). One of the com-ponents of PROSPECT is a Sample Processing and Analysis instrument, known as ProSPA. This minia-ture laboratory is in development with the aims to perform evolved gas pyrolysis, and stepped pyrolysis or combustion, to release volatiles which can be identified and quantified before undergoing isotopic analysis. Also, an ISRU demonstration is to be car-ried out by reducing mineral phases in the presence of hydrogen [2]. Together, these processes will en-hance our understanding of resources available at the south pole and the feasibility of some extraction pro-cesses. The reduction of ilmenite has long been consid-ered a potentially viable technique to extract water, and consequently oxygen, from the lunar regolith with the purpose of producing fuels and life support gases. One of the ISRU demonstration techniques being considered for ProSPA is the reduction of il-menite in the presence of hydrogen. The ilmenite reduction process is well understood, however most experiments are performed on a 100 % ilmenite sam-ple with a continuous flow of hydrogen. The ProSPA instrument is limited to a static system and will likely obtain samples with low concentrations of ilmenite. It is the purpose of this study to determine the extent to which the reaction will proceed in the ProSPA system. Hydrogen Reduction of Ilmenite: Ilmenite, FeTiO3, is a member of the oxide family of minerals, which are the second most abundant minerals on the Moon after silicates [3]. The ilmenite reduction pro-cess goes as follows: FeTiO3 + H2 →Fe + TiO2 + H2O The process requires relatively low temperatures (< 900 ⁰C) compared to oxygen extraction from sili-cates (> 1100 ⁰C), and hydrogen has been shown to be the preferred reducing agent over CH4, CO, and electrolysis [4]. In order to demonstrate the feasibility of the ex-traction process in-situ, ProSPA will heat a ~ 30 mm3 regolith sample in the presence of hydrogen up to 900 ⁰C in one of its ovens. Any water produced will collect at a ‘cold finger’ before being analysed in the ion trap mass spectrometer. A breadboard model is in development at The Open University to trial the reac-tion and determine its feasibility for use on ProSPA. Experimental Set-Up: The breadboard model is comprised of ‘off the shelf’ parts and the layout is shown in Figure 1. The furnace will be operated at ~ 900 ⁰C and the cold finger will be operated at     ~ -190 ⁰C. The connecting pipes will be heated to > 100 ⁰C to inhibit condensation of water between the furnace and the cold finger.  Figure 1 Schematic diagram of the ISRU demonstra-tion model. Before any experiments are run, the system is to be cleared of contaminants by placing it under vacu-um and heating to > 100 ⁰C. A control reading will also be taken from the mass spectrometer to deter-mine the quantity of any remaining contaminants. Similarly, a control reading of a lunar analogue with-out any ilmenite will be taken, to determine the quan-tity of any water that is produced from sources other than ilmenite. Next, simulants doped with ilmenite will be used and the quantities of water produced will be detected. As the concentration of ilmenite is varied, so will the quantity of hydrogen used in the system. The mini-mum amount of hydrogen will be used to avoid in-creasing the pressure and limiting the movement of any water produced. This equates to one mole of hydrogen for each mole of ilmenite present. A number of experiments are envisaged, as well as varying the concentration of ilmenite, such as var-ying the grain size distribution of ilmenite to deter-mine any effect upon the reaction rate. The number of moles of hydrogen will also be increased to see at which point the movement of water is inhibited.  Reaction rate predictions: If a sample with 1% ilmenite present completely reacts, the quantity of hydrogen required for the reaction also increases as shown in Table 1. Considering these values it is pos-sible to determine the gas flow type within the sys-tem. Gas flow can be characterised by its Knudsen number, kn, which considers the mean free path of particles, λ, and the radius of the pipes, a, as kn=λ/a. With a pipe radius of 2 mm, the Knudsen number is in the viscous flow region suggesting the water mol-ecules produced must diffuse through hydrogen be-fore collecting at the cold finger.  Table 1 The effect of ilmenite concentrations on the quantity of  hydrogen required and diffusion time of the water molecules produced. Concentra-tion of ilmenite (%) Mass of hydrogen required (μg) Pressure of hydro-gen in system (Pa) Time for diffusion of water mole-cules (s) 0.5 3.0 731 0.5 1 5.9 1500 1 2.5 14.8 3700 2.5 5 29.6 7300 5 10 59.2 15000 11 50 296.1 73000 51 100 592.1 150000 101  As water molecules are produced they prevent hydrogen from accessing the remaining ilmenite grains and therefore must diffuse through the system before the reaction can continue. The rate of diffu-sion can be calculated with Fick’s law, JAZ=-DAB * dCA/dz, where JAZ is the molar flux of gas A into gas B in the z direction, DAB is the diffusion coefficient, CA is the concentration of gas A, and z is the distance of diffusion [5, 6]. Applying this equation with the assumption that all of the 1 % concentration ilmenite present has reacted; all the water molecules produced are stored in the furnace volume; and the distance between the furnace and cold finger is 0.5 m, the molar flux of water is 0.23 mol m-2 s-1. The velocity of diffusion is therefore shown to be ~ 0.5 m s-1 by dividing the molar flux by the average concentration, resulting in a diffusion time of 1 s. Table 1 shows how the diffusion time increases as the quantity of ilmenite, and therefore quantity of water produced, also increases. There are limits to this analysis due to the assumptions stated above. In reality there will be pressure changes as the water condenses, and the temperature will differ at each end of the system, both of which will significantly affect the diffusion rate.  The other rate controlling steps being considered are the diffusion of hydrogen in the product layer, and the chemical reaction at the interface. It has been shown that for a 100 % ilmenite sample, with a con-tinuous flow of hydrogen (to reduce the effect of water molecules surrounding the sample), the reduc-tion reaction completes in ~ 20 minutes and is con-trolled by the chemical reaction at the interface [7]. This suggests that with the diffusion rate of water also being considered, the time taken for the reaction to complete will be increased further. It is not yet known how the ilmenite concentration in the soil will affect the reaction rate. Further considerations:  The grain size distribution of the lunar soil that is to be obtained from the Luna-27 landing site is not known, and it is understood that grain size distribu-tion does affect the reaction rate of the ilmenite re-duction process. If the grain size is too large then it would suggest that further beneficiation would be needed by future ISRU technologies in similar loca-tions.  In the lunar mare regions, ilmenite can be present at relatively higher abundances, from 10 – 20 vol% [8] whilst the highlands are relatively depleted with an average concentration of < 1 vol% [9]. The min-eralogy of the lunar poles is largely unknown as no direct measurements have been taken to date. How-ever, based on remote sensing data, it is expected that the lunar poles will be mostly comprised of high-lands-type material which may strongly limit the ap-plications of this resource extraction technique, ne-cessitating alternative considerations. On the other hand, if this process was applied on the ProSPA in-strument it would give a ground truth indication for the ilmenite concentration at the lunar south pole and combined with the results of its other experiments, it will help to determine if the south pole is a viable location for resource extraction. Acknowledgements: The funding by STFC of a studentship for H.S. is acknowledged. PROSPECT is a programme of and funded by the European Space Agency.  References: [1] Fisackerly, R. et al. (2016) 9th Symposium on Space Resource Utilization, 0224. [2] Barber, S. J. et al. (2017) ProSPA: the Science La-boratory for the Processing and Analysis of Lunar Polar Volatiles within PROSPECT, Lunar and Plan-etary Sci. Conf, 2171. [3] Heiken, G. H. et al. (1991) Lunar Sourcebook: A user’s guide to the Moon, Cambridge University Press, 122. [4] Gibson, M. A. and Knudsen, C. W. (1985) Lunar bases and space activities of the 21st century, 543-550. [5] Geankoplis, C. J. (1993), Transport processes and unit operations. 3 ed. PTR Prentice-Hall. [6] Delchar, T. A. (1993) Vacuum physics and tech-niques, Chapman and Hall. [7] Zhang, G. H. et al. (2012) ISIJ international, 52(11) 1986-1989. [8] Taylor, L. A. and Carrier, W. D. (1993) in Resources of near-Earth space, University of Arizona Press. 69-108. [9] Taylor, L. A. et al. (2010), JGR: Planets, 115 (E2). 

Hydrogen reduction of ilmenite as an ISRU demonstration for ProSPA

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Hydrogen reduction of ilmenite as an ISRU demonstration for ProSPA

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