The most important fusion method for x-ray fluorescence  (XRF) analysis involves the fusion of a test portion of a prepared sample with lithium metaborate/lithium tetraborate at a temperature between 1000c and 1200c, depending on the material type, in a platinum/5%gold crucible. The flat glass disc is produced using a casting procedure and is presented directly to the XRF for elemental analysis. By using a fused glass disc, the sample is homogenised and particle size effects are eliminated.

 OTHER FUSION TECHNIQUES FOR XRF AND ICP ARE MENTIONED.

 GENERAL NOTES

-1. PRE - TREATMENT 1.1

              Sample PREPARATION

:Samples should be ground to a particle size of < 100 µm 

1.2  PRE-Oxidation:

 The risk of alloying of platinum during fusion is considerable and could be followed by the destruction the crucible. Pre-oxidation is done to reduce this effect it is used before fusion when the sample contains reduced or reducible species such as organics, metallics (Fe, Al, MG Cr, CU, Pb, and Zn and their alloys e.g. Ferro-chromes and sulphides and phosphides.  These can be oxidised in-situ. The quantity of oxidant should be 4-5 times the weight of non-reduced species. One technique is to mix the sample carefully with the oxidant and place this mixture in a hollow in the bed of flux placed in the bottom of the crucible – more flux is placed on top. Avoid contact of the sample with the platinum and never increase the temperature of oxidation above 845 ° C.Time of oxidation: average 3-5 minutes.Nitrides and borides are difficult to analyse and must never come in contact with platinum or its alloys.

 2. THE FUSION

2.1
RANGE OF DILUTIONS

Samples to flux ratios from 1 to 4, up to, 1 to 10 are in common use. The final weights depend on the diameter and thickness of the glass disc. For difficult samples e.g. Ferro-alloys very high dilution must be used.

2.2 
RELEASE AGENTS (Additives):
Ammonium or lithium iodide or bromide to increase surface tension of the melt (reduce the viscosity??)  of the melt. Helps mixing and pouring/cleaning characteristics. (in general 0.5% of the total weight of flux???) These additions should be kept to a minimum e.g. <20 mg of iodine in a 10gm glass disc. Note: Release agent tablets are available: AMMONIUM IODIDE TABLETS    (supplied in bottles of 1500 tablets) Used extensively as a releasing or cleaning agent.

Average weight	21.4 mg
Max variation	+  5%
Composition	66.6%   ammonium iodide 33.4%   starch

Generally 4 minutes in Melting and 3 minutes in Swirling mode and 3 minutes cooling.

 2.4 Swirling
is highly recommended to obtain a good homogenisation of the melt and to reduce the fusion time.  For fully oxidised samples mix the sample and flux before fusion. 

2.5 Casting dish-moulds
to be heated before casting usually at lower temperature than the fusion – usually about 50-100c below crucible temperature. In general fusion temperatures of 1050°C for borate fluxes and 950 °C for metaphosphate flux.  The Phoenix has separate burners for moulds, this allows heavy-duty moulds (typ 100 gm) to be used and individual control of each burner temperature.

 2.6 Cooling.
Forced air for 2 minutes - this can be varied as required. 2 stage (bi-level) cooling is standard on all Phoenix models.  Never handle the bead before total cooling to avoid cracking.

Bubbles in the bead:
Hydrated or hygroscopic flux: use a prefused anhydrous flux.
Decomposition of volatile compounds such carbonates, hydrates, nitrates, sulphides, halides etc. ... with insufficient heating and swirling during the fusion.
In general bubbling is observed during the fusion, it is necessary to heat slowly to avoid losses out of the crucible and consequent loss of material during the fusion time.
Heat till no more gas bubbles appears.
If the temperature of the melt is too low when pouring into the casting dish it may resulting in the absorption of bubbles in air.
Increase the Melting temperature, rocking and swirling, use a flux with a lower fusion point, and add halides to decrease the viscosity of the melt.


Beads sticking in the cast-dish
Add halides such as lithium iodide or bromide
Inspect the surface of the cast dish, polish if dull or scratched.

Irregular analytical results:
For iron, silicon, i.e. this is often due to a poor and therefore non-homogeneous dissolution.Increase the heating, rocking and swirling, use a flux with a lower particle size a grade G, and add halides to decrease the viscosity of the melt.
If a problem still remains change the dilution rapport between the specimen and the flux.

Crystallisation:
Generally due to a poor dissolution.

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Automated Fusion Technology P/L
P.O. Box 808
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Technical service information is issued as a guide to the properties and applications of chemical products sold by Automated Fusion Technology Pty Ltd or XRF Scientific Ltd.
We hope that the information will be of value, and upon request, we shall be glad to supplement it in any way possible.
Every care is taken in compiling this information but we can assume no responsibility for any liability incurred, either in regard to results obtainable or patent infringement

APPLICATION NOTES

MATERIAL TYPES

HIGH IRON

High Iron contents 40 up to 98 % FE203 content (Sinter, hematite, steel)

Oxidation: Is highly recommended (cf. general method).

Recommended fusion temperature: 1200 °C (1350°C temperature of flames)

Recommended flux:

XRFS 17/5 G and Fluore-X 17/5 Sr G (when the sample contains reduced Iron)

XRFS 5050 (when dosage of sodium is not requested) Fluosid 21, XRFS 119

Phoenix Time settings: times depend on fusion mass.

Pre-melt 70 sec. Melting 80 sec. Swirling 280 sec. Cooling 160 sec. Mould delay 80 sec.

 

Ferro-alloys  (cfr 2.13 for more details)

Recommended PARTICLE SIZE of the sample: below 100 µm preferably 63 µm.

A good mix of lithium carbonate/nitrate. Sample and a part of flux are placed on a bed of flux.

Temperature of fusion: 1200-1300 °C.  A preliminary oxidation is necessary with lithium nitrate at 800°C or with a mix of strontium nitrate and lithium nitrate 50 %, 50 %.

On Pre-Melt  mode the temperature is set to 845°C, duration  5-10 minutes.

Caution: it is highly recommended to check if the oxidation step is completed.  For most cases we recommend first to mix the sample with oxidant in the platinum crucible and place this mixture overnight in a well-ventilated muffle furnace.

Flux type:   XRFS ATL100, XRFS ATL100G,

Gas burners: Temperature 1450 °C. (Temperature of the flames)

Alternatives:

XRFS 65 and XRFS 65 G, XRFS 35, XRFS 50, XRFS 20 (Temperature of flames 1350°C)

Additives: Lithium iodide or bromide, potassium or sodium iodate (when the dosage of these elements is not overlapping)

Oxidant: XRF Scientific oxidant mix, lithium, strontium, potassium, sodium nitrate, and sodium perborate. Vanadium pent oxide.

Lithium carbonate is used as an initiator for the decomposition of nitrates.

 

Note:  The method described by R. Rutherford is also highly recommended:

e.g. fuse approximately 2/3 of the flux in the crucible to produce a protective wall. After cooling, introduce the sample mix with oxidant (lithium nitrate with the remaining part of flux) and carry on the oxidation in a muffle furnace at 600°-800°C overnight then fuse as usual.  (It would also be possible to oxidise the Ferro-alloys directly on the Phoenix machine using the protective wall method as described in section 2.13.2)

HIGH CALCIUM

Clinkers, cement

No oxidation step will be necessary except when the sulphur determination is requested.

Flux type: ATL100, XRFS 65, XRFS 15 (contain 15 % of La2O3 that helps with the formation and stabilisation of glass, but reduces the intensity of the emitted X rays)

Caution: in some cements, Alumina from an electrolysis process is produced which may contain some aluminum metal that could destroy the platinum labware.  In this case add 5 % nitrate ( Na, K, Sr) and mix with the flux.

Dilution: 2GM sample plus 10GM flux for a 40 mm diameter bead.

 

((Determination of sodium in cement.  Use XRFS-65.  Temperature of main melting set at 1250°C (Temperature of flames). Timer settings: Main Melting 3 minutes, Swirling 2.5 minutes.))

 

Cement flour , carbonates

For flour the use of a tall size crucible is highly recommended as the decomposition of carbonates induce foaming that may spill over the top of the crucible.

Use a pre-oxidation step of 10 minutes at 850°C in a muffle furnace followed by fusion at 1250°C. (Temperature of flames).  Addition of 2 drops of a solution of lithium iodide or bromide at 250 mg/litre is helpful in some circumstances.

 

Determination of sulphur

It is possible to analyse the total sulphur content of the specimen. The use of a low temperature flux such as XRFS 65  (875 °C) with an oxidant such lithium nitrate permits to transform the volatile sulphides to sulphates.  Sulphides could be measured by difference.

 

The best method is to mix the specimen with 5 times more lithium nitrates as the expected amount of sulphide in the sample and to heat gradually the crucible to 800 ° C for 3-5 minutes and then to fuse as usual.

Sometimes the addition of a small quantity of vanadium pentoxide (0.5 - 2 %) could be useful but of course a reduction of intensity will be observed and therefore not recommended in some cases.

 

(Sulphur in cement)

 For sulphur determination in cement use XRFS 65. The usual level of sulphide is about 0.35 % in cement. Add 200 up to 400 mg of lithium nitrate and mix with the sample. Use Oxidation step at 850°C for 3-4 minutes in a muffle furnace prior to fusion.  Fuse at maximum 1000°C (1150°C temperature of the flames) to avoid the loss of volatile SO2.  Add 2 drops Lithium iodide or bromide (250 mg/litre).

Dilution: 1GM sample, 10GM XRFS 65 for beads of 40 mm diameter))

 SULPHIDES

 Antimony, lead  sulphides:

Ratio:   XRFS 65 = 8GM   Sample = 0.35gm  Oxidant lithium nitrate = 1.250 mg

Premelt on Phoenix fusion machines (900°C) 4 minutes then fuse at 1000°C for 7 minutes.

 

GEOLOGICAL

Most of geological sample could be fused with XRFS 65 or 65 G

Temperature of fusion 1100°C -1200°C

Clays, geological samples, aluminised cement

XRFS 65.      XRFS 65 G (if the sample contains high amount of SiO2).

XRFS 35.      XRFS 20.

Quartzite sand (pure SiO2)

 Flux types:     XRFS 7030.      XRFS 65.     XRFS LiF10.

The heating temperature should be high typically 1200 °C.

Fast swirling is important as the SiO2 tends to concentrate in the bottom of the crucible and thus are difficult to dissolve.  (Recommended Phoenix speed 60 %)

Note.  In older automatic fusion machines with gas/air only burners, even equipped with special crucibles, increases in the rocking will not dissolve all of the silica using a standard mixture of prefused anhydrous flux.  (particle size  between 100 and 500 µm)

XRF Scientific has developed new fluxes with varying particle size constraints. the use of this g-grade improves the total specific surface of the specimen/sample system and permits the easier dissolution of pure sio2.  the particle size distribution is an important factor when used with gas/oxygen burners (Phoenix machines) in obtaining optimal results.

Flux types:    XRFS 7030G.    XRFS 65G

REFRACTORIES

Due to the nature of low fusible heavy oxides such as Zirconium oxide and alkali low fusible oxide such MgO and the presence of SiO2, Chromium oxide, a well-balanced acido-basic flux with a lower point of fusion should be necessary. The G grades having a lower particle size helps to dissolve easier these oxides by increasing the surface of contact between the sample and the flux.  In some very hard cases, the addition of lithium metaphosphate could be beneficial as the temperature of fusion is lower (660 °C).  The metaphosphate has a tendency to stick in the casting dishes and we recommend to heat them at a lower temperature (typ 500-600 °C) as usually done for the casting of borate fluxes.

Note:  The addition of iodides, iodates (lithium) could be useful.

 Flux types:  XRFS 20, XRFS 35, XRFS 50 and also G grades

Magnesite-Chromite 17 %

Ratio:    XRFS 50 =  7 gm.      Sample = 0.5GM.

Temperature of fusion 1250°C  (1400°C temp of flames).

Duration 15 minutes.

Premelt at 900 C, 180 sec, Melting without swirling 225 sec, Melting with swirling 265 sec, Cooling, 300 sec.  On Phoenix fusion machine

Alumina 60%-30%, Silica  40-50%,

Flux type:   XRFS ATL100

Ratio:    XRFS ATL100 = 8gm.   Lithium carbonate = 1 gm.     Sample = 1.8 gm.

Premelt at 900 C, 180 sec, Melting without swirling 225 sec, Melting with swirling 165 sec, Cooling, 300 sec.  On Phoenix fusion machine

SPECIAL NOTES

For determination of light and minors elements

Flux types:  XRFS 65 HP.   XRFS ATLHP if fusion temperature is above 1200°C and if the sample does not contain a high amount of alumina and silica.

Chromite sand and high chromium content specimen

XRFS ATS100 when the dosage of alkali is not important

FXRFS 5050 (if dosage of K and Na is not requested)

XRFS 65 G, XRFS 35G, and

XRFS ATLMP/Dev mix of ATL100, AML with lithium metaphosphate 

Very difficult samples: cassiterite, zirconium high content etc.

Fluore-X G grades 100-250, F20 G

It is easy to detect if the specimens are not well dissolved by the examination of the glass disc.  Bad dissolution gives crystallisation, cracking of the glass disk, opaque glass beads.

Additive: Lithium bromide, iodide

Specimen containing copper

Flux types:   XRFS 35.    XRFS 50.     XRFS 20.

Add only a very little amount of LiBr if the beads stick into the casting-dish.

G grades can be use if a poor dissolution is observed))

Determination of boron

XRFS ALM, lithium metaphosphate with halide addition.

Carburated alloys

PARTICLE SIZE of the sample:  As fine as possible and below 80 µm.

Flux types:  

XRFS ATL100.     XRFS 50.

XRFS PB680:  A mix of lithium tetraborate with sodium perborate.

XRFS 21.   XRFS 119 with addition of strontium and lithium nitrate and halides.

Oxidation step necessary: ref 1.2 Oxidation.

Ferro-Alloys

Method in muffle furnace ( Roger Rutherford British Steel)

All ferro-alloys to be fused using this technique must finely grind in order to obtain fine particle size, i.e. less than 80 µm

A solid protective wall is prepared, from molten lithium tetraborate, inside the crucible.  The finely ground ferroalloy perfectly mixed with the oxidant mix (XRF Scientific) is added carefully to the crucible, to form a thin layer on the solid wall.  The crucible is placed in the furnace, set at 800C, for a minimum of 3 hr.  This preliminary oxidation stage is important. Furnace design ensures a good airflow through the furnace to provide the necessary oxygen. If time permits the samples are allowed to oxidise overnight.  This maximises the analyst's time. 

Fuse in a muffle furnace set at 1200°C, for 10 min.

After the fusion period the melt is poured into a preheated casting dish.

Weigh 8.000 gm of dried lithium tetraborate (XRFS ATL100) and transfer it into the crucible.  Place the crucible in the furnace set at 1200 °C, for 3 min.  After 3 min using tongs, remove the crucible containing the molten lithium tetraborate.  Holding the crucible firmly with the tongs slowly rotate the crucible.  Continue this crucible rotation to form a layer of lithium tetraborate half way up the inner crucible wall.  As the lithium tetraborate cools and solidifies a wall is formed of uniform thickness.  When the wall has solidified allow the crucible to cool.

Weigh 0.4000 gm of dried ferro-alloy.  Mix well the ferro-alloy with 4.000 g of OXIDANT MIX.  Add the mix of oxidant and ferroalloy to the cool crucible to form a layer in its base.  Place the crucible in the furnace set at 800 °C for a minimum of 3-4 hr. Good air flow through the furnace is essential. 

After the oxidation/dissolution period the crucible is transferred to a muffle furnace set at 1200 °C or a Phoenix fusion machine (flames temperature 1350 °C).   The total fusion time is 10 min.  At 4 and 8 min into the fusion process the crucible is agitated to ensure that the melt becomes homogeneous.   Note this is done automatically in the Phoenix.))

FERRO ALLOYS

((METHOD WITH THE PHOENIX FUSION MACHINE and Protective wall))

Weight 0.200 gm of very finely grounded ferro-alloy sample (100 µ maximum) and mix it carefully with 4 gm of OXYDANT MIX.  Put to one side.

Weight 5.000 gm of XRFS ATL100 and place it in the crucible.

In order to create a wall of flux with the Phoenix machine proceed as such :

First push on the "Hold Swirling button” set Premelt to 10 sec, Melting to 120 sec at 1350°C.

Start the machine. When the flux is totally melted stop the heating process by switching off the Burner Select Buttons for each burner and continue the swirling without flames until we obtain a wall of flux around the inside of the crucible.

Cool down and add the mix in the protected crucible.

Then start the program for ferro-alloy:

Premelt 7-10 min at 800°C, Melting 3 min at 1350 °C, Swirling  4 min. 

Cooling rate must be set depending on the sample.

CAUTION: it is necessary to test the method beforehand in order to check that there will be no damage to the platinum labware.

Ferro-silicium, ferro-manganese, silicium manganese

Weigh 200 mg of finely grounded ferro-alloy mix with 2000 mg potassium carbonate and 1000 mg potassium iodate (KIO3).  Proceed as described previously. (5000 mg XRFS ATL100)

Premelt-pre-oxidation: Heat at 800 °C for 10 min.  We observe a black viscous mobile liquid

Then fuse as described previously.

2.13.3 DIRECT METHOD WITH PHOENIX FUSION MACHINE

2.13.3.1.          Silicium carbide

Prepare a mix of 200 mg sample with 650 mg V2O5 , 1.200 mg XRFS ATL100, 1.200 mg Sr(NO3)2 and 3.600 gm Li2CO3. Add 2 drops of LiBr solution at 250 mg/litre

In the bottom of a tall shape crucible add 4.000 gm of XRFS ATL100.

Pre-oxidation step  : 12 minutes, temperature of flames 900 °C

Melting without swirling: 4 minutes, temperature of flames 1400 °C

Melting with swirling: 2 minutes.

Cooling: 5 minutes.

 

2.13.3.2.          Ferro-Silicon

Prepare a mix of:

200 mg sample.   500 mg V2O5.   1.000 mg XRFS ATL100.    500mg Lithium nitrate.

200 mg Sr (NO3) 2 0.    200 mg of potassium nitrate and 1.600 gm Li2CO3.

In the bottom of a high shape crucible add 4.000 gm of XRFS ATL100.

Phoenix fusion Machine: Pre-oxidation step: 12 minutes, temperature of flames 900 °C.

Melting without swirling: 4 minutes, temperature of flames 1400 °C.

Melting with swirling: 2 minutes.

Cooling: 5 minutes

2.13  Steel industry

2.14.1.Slag, agglomerate, dust furnace, hematite, Iron ore 40 up to 90 % Fe.

Ratio:   XRFS PB680 = 8.500 gm.    Sample = 500 mg.

Phoenix fusion Machine: Premelt 180 sec at 900°C.  Melting 240 sec at 1400°C (temperature of the flames).  Melting with swirling 300 sec.   Forced cooling 300 sec.

2.14.2.Slag (30% Fe, 50% CaO, 10 % Sio2), Dust (50 % Fe, 30% Zn)

Place 6.000 mg of XRFS ATL100 or XRFS 17/5 in the bottom of the crucible add on top a mix of 250 mg of Sample with 1.250 mg Lithium nitrate and 2.000 gm of XRFS ATL100.   On top of this add a lid of 1.000 gm XRFS ATL100.

Phoenix Fusion Machine:   Premelt 420 sec at 900°C.    Melt 240 sec. at 1375°C (temperature of the flames).    Melt with swirling 240 sec. Forced cooling: 320 sec for the slag.

No forced cooling for the furnace dust.

4. General methods used in the steel industry

XRF	BEADS	General	Gas dust	Slags	Fe-Cr- C	Fe-Si
					( 1)	(2)
Sample	Quantity	0,300 g	0,250 g	0,500 g	0,250 g	0,250 g
	Granulo.	< 80 µ	< 80 µ	< 100µ	<200 µ	<200 µ
Flux	Type	XRFS 65	ATL100	ATL100	ATL100	ATL100
	Origin	XRF Scientific	XRF Scientific	XRF Scientific	XRF Scientific	XRF Scientific
	Quantity	5,000 g	5,000 g	5,500 g