Abstract No. 48. Multi-step oxidation of mineral samples by borate fusion Dirk Töwe, HRT Fusion GmbH

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1 Abstract No. 48 Multi-step oxidation of mineral samples by borate fusion Dirk Töwe, HRT Fusion GmbH Service laboratories performing borate fusion sample preparation for XRF analysis are exposed to a very wide range of sample types. New projects or unusual sample composition could be associated with an unusual failure rate during sample preparation. Our presentation outlines the work involved in a sample preparation technique for mineral samples related to the iron ore industry. But the approach could very well be used for a much wider range of samples, even alloys. During the borate fusion process, the oxides are dissolved in a molten flux, and if a fraction of the sample is sub-oxidized, the outcome of the fusion is disappointing: sticky melt, cracked beads, undissolved sample and short Ptware service life. The use of a solid oxidizer along with the flux during the fusion process has been widely documented but often used with mixed results. The decomposition of these salts creates a strongly oxidizing environment and the sample constituents are expected to reach a maximum oxidation state for later dissolution in the molten flux. Using a Nieka G-Series fusion instrument, we investigated key parameters to consider when developing a sturdy and rapid fusion method with an oxidizing agent or with a number of them using a multi-step approach. Using a variety of measurements performed during the fusion process, we show the evolution of the various reactions with time and temperature in the fusion crucible and the outcome of the fusion procedure. This approach can be used as a framework for method development instead of the usual empirical fusion procedures.

2 Multi-step oxidation of mineral samples by borate fusion The fusion experts when only quality matters Borate fusion for sample preparation established procedure fully oxidized sample + flux HRT Fusion GmbH Dirk Toewe Dusseldorf, Germany / March 20, 2015 homogeneous glass bead for X-Ray analysis Sub-oxidized or metallic samples will not dissolve in the flux. 2

3 The case study of a service lab reveals the challenges of processing mineral samples. processing of up to 10,000 samples per month by fusion most samples originate from the iron ore industry or in-house R&D projects very wide range of composition of the samples Fe 2 O 3 and SiO 2 are usually the main components of the samples Challenges high re-processing rate of unknown samples inadequate processing of metallic and partially oxidized samples nearly impossible to process these samples Methodology of the chemists to develop perfect fusion methods selection of the impossible samples from their extensive database development of an all-around fusion approach with an emphasis on the pre-oxidation phase utilization of a fusion instrument that allows a precise control of the pre-oxidation steps Aim Oxidation of the samples on the fusion instrument WITHOUT a preliminary hightemperature treatment in a muffle furnace. 3 4

4 NIEKA Gas fluxer G4 proprietary NIEKA heating chambers NIEKA Gas fluxer premium and innovative fusion instruments Productivity very high throughput due to drastically reduced fusion times perfectly homogeneous samples advanced self-monitoring Accuracy reliable temperature measurement of each individual crucible dedicated mold heating perfect reproducibility of the results Efficiency proprietary heating chambers increase efficiency and reduce energy consumption new line of gas fusion instruments with a very precise control of all fusion parameters fusion instrument allows the development of customized methods very flexible fusion sequence monitoring of the fusion process in real time = easier fine-tuning of the fusion method Usability built-in touchscreen with easy import and export of fusion methods via USB or Internet 5 6

5 temperature Drilling samples, in particular, illustrate the difficulties associated with the fusion method used so far. Previous method pre-oxidation using ammonium nitrate NH 4 NO 3 one layer of flux at the bottom, then sample material, then oxidizer on the top total process time: more than 16 minutes (incl. cooling) New method: multi-step oxidation Difficulties typical re-fusion rate of drilling samples over 30 % using the previous fusion method The fusion instrument previously used does not allow to decompose NH 4 NO 3 very slowly. The efficiency of the oxidation step is limited. broken glass disks, undissolved particles crystallization, vigorous chemical reaction during oxidation selection of a sub-set of samples with a very low success rate using the previous method implementation of a multi-stage oxidation process PRIOR to the main fusion step Multi-step oxidation enables successfull fusion of samples with a very low success rate. Previous method pre-oxidation main fusion steps Multi-step oxidation pre-oxidation 1 no heat pre-oxidation 2 7 main fusion steps With the previous method, pre-oxidation is directly followed by the individual main fusion step(s). Step 2 solely relies on a) the exothermic reaction initiated during the 1 st step b) the energy accumulated in the crucible 8

6 In order to develop the 1 st step of the fusion method, heating patterns were established. insertion of thermocouples at various places in the crucible during the heating process heating patterns vs time C after 60 sec. A temperature A 350 C after 60 sec. flux B B temperature 9 main fusion steps pre-oxidation 2 pre-oxidation 1 no heat Without heating (step 2), the oxidation process is carried out slowly and completes in 60 seconds. We then proceed with step 3 (main fusion steps). The entire step 2 is conducted in the optimal condition to use NH 4 NO 3 : ramping from 220 C.CETAS 180 C to The individual measures applied to the fusion method of the drilling samples Multi-step oxidation time time 10

7 The decomposition of ammonium nitrate NH 4 NO 3 NH 4 NO 3 N 2 O + 2 H 2 O NH 4 NO 3 NH 3 + HNO 3 4 HNO 3 2 H 2 O + 4 NO 2 + O 2 decomposition in the first step After two initial reactions during the early stages of the pre-oxidation, heating is switched off to let these reactions complete slowly. The decomposition of nitric acid (HNO 3 ) then takes place in an additional oxidizing step after resuming the heating. A slow decomposition of ammonium nitrate NH 4 NO 3 is pivotal to a successful fusion since it allows a longer action on the sub-oxidized compounds in the sample. Multi-step oxidation allows a speedy fusion of difficult samples and results in perfectly homogeneous disks. Multi-step oxidation time required: less than 12 minutes (pre-oxidation, fusion, cooling) WITHOUT a preliminary treatment in a muffle furnace result: perfectly homogeneous glass disks (Fe 2 O 3 RSD 0,16 %) utilization of 35/65 (LiT/LiM) + 0,5 % LiBr 11 12

8 The same approach can be applied to samples with high metallic content. identical approach as with the drilling samples selection of a sub-set of samples with high metallic Fe content (75 80 %) and containing carbon up to 5 % Difficulties Those samples were impossible to process with the previous method and fusion instruments used by the service lab. no Loss of ignition but Gain on ignition of 28 % an ignition step in the muffle furnace was mandatory and time consuming Development of two fusion methods for the processing of the most difficult samples of the set. A. One oxidation step only NH 4 NO 3 as oxidizer works well, but only with an extended fusion time (+ 3 minutes) = ~ 15 minutes B. Two oxidation steps utilization of NH 4 NO 3 and LiNO 3 to extend the spectrum of the oxidation process The measured Fe 2 O 3 concentrations showed a low RSD of 0,19 %. Results 13 Both methods are suitable to process difficult samples (such as those with high metallic content). No sample pre-treatment is required. 14

9 Common oxidizers and their decomposition temperature NH 4 NO C NaNO C KNO C LiNO C Sr(NO 3 ) C Deliberate selection of two chemicals which lie further apart in their temperature range. NH 4 NO 3 only oxidizer which can be volumetrically weighed dissolves completely no interfering elements LiNO 3 for the extension of the oxidation process Multi-step oxidation a framework more than a method The pre-oxidation step(s) may be a critical component of your fusion method. Each maker s fusion instrument has a different heating behavior. Understanding your system specifics can help to develop more effective methods. Borate fusion is not a black box: Observation is still an important part of method development and understanding the various inherent steps of the process is the key to success

10 Thank you for your attention! 17