International Conference on Chemical, Mining and Metallurgical Engineering (CMME'2013) Nov. 27-28, 2013 Johannesburg (South Africa)

Production of Anhydrous Tetrafluoride from Plasma Dissociated Zircon and Ammonium Bifluoride

Milton M. Makhofane, Johann T. Nel, Johan L. Havenga, and Ayo S. Afolabi

methods are that the starting materials are expensive and Abstract—Zirconium tetrafluoride is used as a catalyst for hydrogen is extremely hazardous to work with. the production of chlorofluoro-hydrocarbons and to In 2010, Nel, et al. [11] registered a patent for the manufacture zirconium tetrafluoride based glass. The production of zirconium tetrafluoride by reacting plasma traditional methods of producing zirconium tetrafluoride dissociated zircon with ammonium bifluoride. In 2011, involve fluorinating zirconium oxide, chlorides, etc. with Makhofane et al. [12] presented a paper on the production of aqueous or anhydrous . In this work, zirconium tetrafluoride from the method patented by Nel et al. zirconium tetrafluoride was manufactured from plasma [11] at the ZrTa2011 New Metals Development Network dissociated zircon and ammonium bifluoride in a batch reactor Conference. This paper was a follow up on the paper system. It was confirmed by XRD analysis that zirconium presented at this conference and presents the details on the tetrafluoride was produced with a purity of 82.87%. The batch purity of the synthesized zirconium tetrafluoride and efficiency reactor system was 93.85% efficient. of the batch reactor system used. The production of zirconium tetrafluoride in the batch reactor system follows the reactions Keywords—ammonium bifluoride, Plasma dissociated zircon, in Equations 1-4. batch reactor, efficiency. ZrO2.SiO2 + 8NH4HF2→ (NH4)3ZrF7 + (NH4)2SiF6 + 3NH4F + I. INTRODUCTION 4H2O (1) IRCONIUM tetrafluoride together with zirconium oxide (NH4)3ZrF7 → (NH4)2ZrF6 +NH3 + HF (2) Zand partially fluorinated zirconium oxide are being used as (NH4)2ZrF6 → NH4ZrF5 + NH3 + HF (3) a catalyst for the fluorination of chloro-hydrocarbon to NH4ZrF5 → ZrF4 + NH3 +HF (4) chloro-fluoro hydrocarbons [1-3]. Other uses of zirconium tetrafluoride include the synthesis of zirconium fluoride based II. PROCEDURE glass, where this glass exhibit high optical transparency from the near ultraviolet (0.20 μm) to the mid-infrared (8 μm) in the A. Reagents electromagnetic wavelength spectrum. These zirconium Plasma dissociated zircon (ZrO2.SiO2) with a purity ≥ 95% fluoride based glasses are used in the production of laser was manufactured at The South African Nuclear Energy windows, fiber optic elements and Faraday rotators [4-6]. Corporation SOC Ltd. (Necsa) by subjecting zircon to high The traditional methods of producing zirconium temperatures, approximately 2500°C, in the flame of a non- tetrafluoride involve fluorination of zirconium compounds transferred arc plasma reactor. This process was followed by (oxides, chlorides, oxychlorides, hydroxides and nitrates) with rapid quenching of the product to form the plasma dissociated aqueous or anhydrous hydrogen fluoride. These methods have zircon [13]. The plasma dissociated zircon was used without been well documented in the US Patent office since 1953 [7- further treatment. Ammonium bifluoride (NH4HF2) of 10]. However, the major drawbacks to these traditional analytical grade (Sigma-Aldrich, South Africa) was used without further treatment. Milton M. Makhofane is with Plasma Technology, Applied Chemistry, Research and Development, South African Nuclear Energy Corporation SOC B. Process and equipment description Ltd, P O Box 582, Pretoria, 0001, South Africa and Department of Civil and The production equipment consists of a reactor, a cold trap, Chemical Engineering, College of Science, Engineering and Technology, a cyclone, a Fourier Transform Infrared spectrometer (FTIR) University of South Africa, P/Bag X6, Florida 1710, Johannesburg, South Africa. Tel: +27721367377; E-mail: [email protected]. and utilities (Figure 1). The utilities consist of hydrofluoric Johann Nel, Plasma Technology, Applied Chemistry, Research and scrubber, potassium hydroxide scrubber, a vacuum pump Development, South African Nuclear Energy Corporation SOC Ltd, PO Box and a cooling water circuit (not shown in the Figure). The 582, Pretoria, 0001, South Africa reactor is heated externally with a ceramic band heater having Johan Havenga, Plasma Technology, Applied Chemistry, Research and Development, South African Nuclear Energy Corporation SOC Ltd, PO Box a maximum power output of 2 kW. The reactor has a height of 582, Pretoria, 0001, South Africa 200 mm, a diameter of 100 mm and is fabricated using a 316 Ayo Afolabi is with the Chemical Engineering Department, University of stainless steel. The lid of the reactor is fitted with a type K South Africa, P/Bag X6, Florida 1710, Johannesburg, South Africa.

213 International Conference on Chemical, Mining and Metallurgical Engineering (CMME'2013) Nov. 27-28, 2013 Johannesburg (South Africa) thermocouple, a nitrogen inlet and a pressure gauge. The cold for thermogravimetric analysis (TGA) and x-ray powder trap is fitted with a water cooled copper coil. The temperature diffraction analysis (XRD). The cold trapped and the cyclone of cooling water was maintained at 18°C by cooling water were also opened and the waste product was collected, mixed, circuit. weighed and taken for XRD analysis.

Nitrogen V-11 inlet III. RESULTS AND DISCUSSION

V-10 Water inlet Water outlet The X-ray powder diffraction analysis confirmed that the

PI P01 I-5 P03 reactor product contained anhydrous zirconium tetrafluoride FI I-4 P-1 with a monoclinic crystal structure. The spectrum of the x-ray Temperature readout powder diffraction of the reactor product is depicted in Fig. 2.

V-7 TI PI P05 I-2 I-3 35000 34000 33000 32000 31000 30000 P02 29000 28000 27000 ZrF4 pattern 26000 25000 Cold 24000 V-8 23000 V-9 Trap Cyclone 22000

21000 P06 P04 20000 19000 18000 17000 TC V-6 V-5 01 16000 15000 Lin (Counts) 14000 Reactor 13000 12000 11000 10000 9000 8000 7000 6000 5000

P07 4000 3000 2000 PI To 1000 I-1 atmosphere 0

Orifice 01 P13 10 20 30 40 50 60 70 80 90 100 110 120 130 140 HF KOH 2-Theta - Scale P10 P11 P12

P08 File: ZrF4_Run2 product_b.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 144.991 ° - Step: 0.020 ° - Step time: 94.5 s - Temp.: 25 °C (Room) - Time Started: 17 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00 ° - Scrubber Scrubber Operations: Strip kAlpha2 0.500 | Background 0.014,1.000 | Import

P09 00-033-1480 (*) - Zirconium Fluoride - ZrF4 - Y: 61.47 % - d x by: 1. - WL: 1.5406 - Monoclinic - a 9.55850 - b 9.95120 - c 7.70710 - alpha 90.000 - beta 94.520 - gamma 90.000 - Body-centered - I2/a (15) - 12 - 730.8 E-1 Outside V-4 V-2 Utilities Fig. 2 Powder XRD diffraction pattern of the reactor product overlaid V-1 V-3 with zirconium tetrafluoride pattern

IR Cell Nitrogen purge The x-ray powder diffraction analysis of the mixed waste product from the cold trap and the cyclone confirmed that the Fig 1 Schematics of zirconium tetrafluoride production system waste product contains ammonium fluoride, as shown in Figure 3, and the ammonium hexafluorosilicate bounded with C. Manufacturing of zirconium tetrafluoride ammonium fluoride, as revealed in Fig. 4. In a typical experiment for the manufacturing of zirconium 27000 tetrafluoride, 100 g of plasma dissociated zircon and 200 g of 26000 25000 ammonium bifluoride were mixed together. The mixture was 24000 23000 NH4F pattern placed in the reactor and the latter was closed. The reactor was 22000 -1 21000 first purged with nitrogen at 0.25 g.s and then the system was 20000 19000 evacuated to a pressure of 78.8 kPa (atmospheric pressure = 18000 17000 86.8 kPa) and maintained at that pressure. The flow of the 16000 15000 cooling water, at room temperature, for the cold trap was 14000 -5 3 -1 13000 maintained at 8.33x10 m .s . The temperature of the reactor 12000

Lin (Counts) 11000 was slowly increased to reach an internal temperature of 10000 9000 150°C and maintained at that temperature for thirty minutes to 8000 7000 allow the fluorination of plasma dissociated zircon to take 6000 5000 place. The temperature was increased so that the internal 4000 3000 temperature reached 280°C and maintained at that temperature 2000 1000 for thirty minutes to sublime the ammonium fluorosilicate. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Lastly, the temperature was increased to reach an internal 2-Theta - Scale File: ZrF4_Run2 cold finger_a.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 144.991 ° - Step: 0.020 ° - Step time: 94.5 s - Temp.: 25 °C (Room) - Time Started: 20 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00 ° temperature of 380°C and maintained at that temperature for Operations: Strip kAlpha2 0.500 | Background 0.145,1.000 | Import 00-035-0758 (*) - Ammonium Fluoride - NH4F - Y: 18.16 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 4.44080 - b 4.44080 - c 7.17260 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 122.4 thirty minutes to decompose the formed ammonium fluorozirconate compounds. After the experiment, the reactor was allowed to cool to room temperature over a period of 12 Fig. 3 XRD powder diffraction pattern of cold trap product overlaid hours. with ammonium fluoride pattern The product was collected after the reactor has cooled to room temperature. The reactor product was weighed and sent

214 International Conference on Chemical, Mining and Metallurgical Engineering (CMME'2013) Nov. 27-28, 2013 Johannesburg (South Africa)

27000 26000 25000 24000 23000 22000 21000 20000 19000 18000 17000 (NH4)2SiF6.NH4F pattern 16000 15000 14000 13000 12000

Lin (Counts) 11000 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 2-Theta - Scale File: ZrF4_Run2 cold finger_a.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 144.991 ° - Step: 0.020 ° - Step time: 94.5 s - Temp.: 25 °C (Room) - Time Started: 20 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00 ° Operations: Strip kAlpha2 0.500 | Background 0.145,1.000 | Import 00-003-0097 (I) - Ammonium Silicon Fluoride - (NH4)2SiF6·NH4F - Y: 14.69 % - d x by: 1. - WL: 1.5406 - Tetragonal - a 8.04000 - b 8.04000 - c 5.84500 - alpha 90.000 - beta 90.000 - gamma 90.000 - Primitive - P4/m

Fig. 4 XRD powder diffraction pattern of cold trap product overlaid Fig. 5 Thermogravimetric analyzer scan of the reactor product with ammonium fluorosilicate pattern About 68.78 g of zirconium tetrafluoride was present in the The material balance of the experiment is stated on Table I. reactor product collected (83.00 g). According to Equation 5, The material balance of the experiment was determined by the zirconium tetrafluoride formed in this experiment had a subtracting the product collected from the reactor, the cold purity of 82.87%. trap and cyclone from the mass of the reactant initially placed into the reactor at the beginning of the experiment. The (5) balance of 138.60 g could be identified as the gaseous product released (H2O, NH3 and HF) during the experiment, as The theoretical mass of zirconium tetrafluoride expected observed by FTIR, and the solid particles entrained in the from 80.34 g of plasma dissociated zircon, according to stream of the nitrogen used to purge the reactor. Equation 6, is about 73.29 g.

(6) TABLE I MATERIAL BALANCE OF THE EXPERIMENT Description Amount (g) Reactant placed into reactor 300.00 For this particular experiment, the process efficiency seems Reactor product 83.00 to be 93.85% for the production of zirconium tetrafluoride. Mixed product from cold trap and 78.40 cyclone Balance 138.60 (7)

The starting point temperature of the weight loss of the reactor product sample during the thermogravimetric analysis IV. CONCLUSIONS corresponds closely to the sublimation temperature of This study has shown that a high purity zirconium zirconium tetrafluoride (T = 600°C). It is seen in Fig. 5 that tetrafluoride can be produced by reacting plasma dissociated no weight loss was experienced at temperatures below zircon with ammonium bifluoride as confirmed by XRD 588.27°C, this indicates that all the ammonium fluorozirconate results. The batch reactor used in this production was able to compounds has decomposed to form zirconium tetrafluoride. synthesize zirconium tetrafluoride with a purity of about The reactor product experienced a total weight loss of 82.87% 82.87% and the efficiency of the batch reactor was about between the temperature range of 588.27°C and 790.05°C. 93.85%. According to Equation 1, 100 g (0.5455 mol) of plasma dissociated zircon will react completely with 248.95 g (4.3643 mol) of ammonium bifluoride to form ammonium ACKNOWLEDGMENT fluorozirconate, ammonium fluorosilicate, ammonium fluoride The authors wish to acknowledge the technical supports of and water. In this experiment, ammonium bifluoride was the the South African Nuclear Corporation SOC Ltd, and Messrs limiting reactant since 200 g (3.5061 mol) was used. The Andrew Pienaar and Tshepo Ntsoane for conducting TGA and 200 g of ammonium bifluoride will react with 80.34 g XRD analyses. The financial support of the Department of (0.4383 mol) of plasma dissociated zircon, thus 24.48% excess Science and Technology (DST) of South Africa through the of plasma dissociated zircon was used. Advanced Metal Initiative (AMI) is also appreciated.

215 International Conference on Chemical, Mining and Metallurgical Engineering (CMME'2013) Nov. 27-28, 2013 Johannesburg (South Africa)

REFERENCES based plasma processes for the manufacturing of fluorocarbon polymers. He is currently the grant holder of several research projects which were funded by [1] S. M., Darling, “Conversion of hydrocarbons in the presence of a the Innovation Fund of South Africa, the Technology Innovation Agency, the catalyst comprising alumina and an oxyfluoride of beryllium, titanium, National Research Foundation and the Department of Science and zirconium” US Patent 2449061, 14 Sept.1948. Technology as well as various contract research projects. Dr Nel is the [2] S.M., Darling, Supported metal oxyfluoride cracking catalyst, US Patent national coordinator of the New Metals Development Network of the 2524771, 10 Oct. 1950. Advanced Metals Initiative of Department of Science and Technology. This [3] T. Tanuma, H., Okamoto, K., Ohnishi, S., Morikaw, and T., Suzuki, programme focuses on the development of new processes for the beneficiation “Activated zirconium oxide catalyst to synthesize of zirconium, hafnium, tantalum and niobium by means of plasma and dichloropentafluoropropane by the reaction of dichlorofluoromethane fluoride technology. He often acts as study-leader or co-study leader for post with tetrafluoroethylene” Appl. Catalysis A: 359, 2009, pp.158-164. graduate students at several universities in South Africa. He has published or [4] M. Poulain,“Halide Glasses” J. Non-Crystalline Solids 56, 1983, pp. 1- co-published more than 20 peer reviewed articles, made more than 40 14. conference contributions and held six patents. [5] R.H. Nielsen, J.H. Schlewits and H. Nielsen. “Zirconium and zirconium compounds” in: Kirk-Othemer Encyclopedia of Chemical Mr. Johan L. Havenga has more than 33 years experience in the R&D Technology, Vol. 26, 5th Ed, New Jersey, 2004, p. 638. division of The South African Nuclear Energy Corporation SOC Ltd. He has [6] M. Robinson, R.C. Pastor and M. Braunstein “Fluorozirconate, glass vast amount of experience working with high temperature Plasma and the process for making the same” US Patent 4341873, 27 Technologies, Nano Materials, Uranium Chemistry, Fluidized Beds, Spray July 1982. Driers and Rotating Oven. [7] W.J.S. Craigen, G.M. Ritcey, E.G., Joe, F.W. Melvanin, and B.C. Smart “Zirconium tetrafluoride process” US Patent, 3 702 883, 14 Nov. 1972. Prof. Ayo Samuel Afolabi obtained his BSc (Hons) and MSc at The [8] O.F. Sprague. “Preparation of zirconium tetrafluoride” US Patent, Federal University of Technology Akure, Nigeria in Metallurgical and 2789882, 23 April 1957. Materials Engineering. He completed his PhD at the University of the [9] H.A. Wilhelm and K.A. Walsh “Method of producing zirconium Witwatersrand Johannesburg, South Africa specializing in Carbon tetrafluoride” US Patent, 2602725, 08 July 1952. nanotechnology and fuel cell. His research interests are in carbon [10] H.A. Wilhelm and K.A. Walsh. “Preparation of zirconium nanotechnology, fuel cell technology, materials characterization, corrosion tetrafluoride” US Patent, 2635037, 14 April 1953. engineering and extractive metallurgy. [11] J.T. Nel, W. Du Plessis, P.L. Crouse and W.L. Retief. “Fluorinating zircon to zirconium tetrafluoride” Republic of South Africa Patent, ZA 2009/05297, 29 July 2010. [12] M.M. Makhofane, J.L. Havenga, J.T. Nel, W. Du Plessis and C.J. Pretorius. “Manufacturing of anhydrous zirconium tetrafluoride in a batch reactor from plasma dissociated zircon and ammonium bifluoride, ZrTa2011” New Metals Development Network Conference, 12-14 Oct. 2011, Magaliesburg, South Africa. [13] J.L. Havenga, and J.T. Nel. “The manufacturing of plasma dissociated zircon (PDZ) via a non-transfer arc process by utilizing 3 x 150 kW DC plasma torch, ZrTa2011” New Metals Development Network Conference, 12-14 Oct. 2011, Magaliesburg, South Africa.

Mr. Milton Makhofane is a registered M. Tech student in Chemical Engineering Department, University of South Africa (Unisa). He was an independent contractor at this institution in 2006 and 2007 as a peer collaborative learning facilitator for first year Engineering Mathematics (MAT181Q) for National Diploma Engineering Students and in 2008 as a Chemistry I Practical Demonstrator for BSc students. He currently holds a position of Technician (Engineering) at NECSA from 2010 to date. He has published some papers in peer reviewed journals and was awarded The Final Year Honour/Bech DST/NRF Innovation Scholarship from the National Research Foundation (NRF) in 2011. He is currently registered as a Candidate Engineering Technician at the Engineering Council of South Africa (ECSA), also as a Graduate Member at the SA Institution of Chemical Engineers (SAIChE) and as an Associate Member of The Southern African Institution of Mining and Metallurgy.

Dr Johann T Nel (Pr.Sci.Nat.) finished his MSc degree (Cum Laude) in of rhodium complexes at the University of the Free State in 1982. In 1983 he started his professional career as a Nuclear Research Scientist at the erstwhile Uranium Enrichment Corporation of South Africa (UCOR). He was involved in several projects including uranium and fluoride chemistry, nuclear materials, gas separation membranes, surface and interface analysis, chemical cleaning and plating, nuclear decontamination, analytical chemistry, etc. In 1990 he obtained his PhD in Chemistry at the University of Pretoria with the title “Surface analysis of uranyl fluoride layers by glow discharge optical emission spectroscopy”. In 1995 he became Programme Manager of the Metal Oxide program at the South African Nuclear Energy Corporation Ltd (Necsa). He developed and co-developed several chloride based and fluoride based mineral beneficiation processes for zircon, tantalite, niobium, titanium, silicon, aluminium, etc. Several of these processes were up-scaled to pilot or semi-commercial scale. As Manager of Plasma Technology at Necsa since 2000, he was also responsible for the development of nano-sized products which could be produced by means of direct current non-transfer arc plasma systems. He was also the Project Leader for fluoride

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