FLUX
SELECTION CRITERIA
Introduction
X-rays
fluorescence analysis is a powerful analytical tool essentially based on
comparison with standards. It’s why the accuracy and the repeatability of XRF
analysis depends to a large extent upon the sample preparation.
The
Fusion
The
fusion is based on a very old and very well known analytical technique: the
pearl of borax. It consists to dissolve oxides in a flux, commonly a
borate, to obtain a vitreous homogenous glass disc after cooling (a bead).
This
technique is superior (to press pelletizing) for two major reasons. First fusion
destroys the mineral grains and their structurs, thus distributing constituent
elements homogeneously throughout the mass and eliminating the grainsize effect
on mass absorption. Second, dilution greatly reduces errors introduced by mass
absorption (inter-element effect).
The
fusion is applicable to the majority of elements having an atomic number higher
than 10, The state of surface of the fused disc is clean and perfectly
flat.
Samples
and standards could be stored during relatively long period.
Fusion
gives very high accurate and reproducible analysis results.
Properties
and characteristics
The
most used fluxes are:
FLUX
LT100:
Di-lithium tetraborate, Li2B4O7
or Li2O, 2 B2O3
- melting
point : 920°Celsius.
Lithium
tetraborate which reacts easily with alkaline oxides such as :CaO, MgO, K2O,
Na2O is called an “ acidic ”
flux.
Lithium
tetraborate is the most largely used fluxes and the cheapest and has a wide
range of application: steel, cement, geological etc. and gives excellent and
stable glass disks.
FLUX
LM100:
lithium metaborate, 2Li2O,2B2O3
- melting
point : 845 °Celsius.
Lithium
metaborate, which reacts more easily with : Al2O3,
SiO2, P2O5,
sulphates, etc., is a called for this raison a “ basic ”
flux.
Lithium metaborate has a lower
fusion point and is more hygroscopic than lithium tetraborate.
Lithium
metaborate is never used alone for beads production, but mixed with lithium
tetraborate it permits to extend the range of soluble oxides and their rate of
dissolution. This product is often used to prepare AA
and ICP solutions by the fusion technique.
FLUX
ST100:
di-sodium tetraborate, Na2B4O7
or Na2O, 2 B2O3 - melting
point :740°Celsius
Less
acid than lithium
tetraborate
Sodium
tetraborate is less and less used because contains more alkaline products, Ca,
Mg, K. However in combination with lithium tetraborate it is a excellent flux to
dissolve metallic oxide Cr, Ni, Fe.
FLUX
LT65:LM35 (UNIVERSAL
FLUX) - melting
point : 875 ° Celsius
a
homogeneous blend of 66 % Li2B4O7
and 34 % LiBO2, has amphoteric
characteristics
A
typical mixture of 66 % Li2B4O7
and 34 % LiBO2
is recommended for alumina containing products such as cement, geological
samples, rocks etc.... We produce a prefused
homogeneous mix of above composition :the FLUX LT65:LM35
FLUX
LT35:LM65 is recommended for fusing aluminosilicates, aluminas, bauxites, iron
ores,...
Selection
of fluxes based on the sample composition.
According
to the sample composition
to be analysed one can select qualitatively the type of flux or fluxes to be
selected.
For
a product containing non-metallic oxide lithium tetraborate should be a good
choice, but if these product contains alumina or/and silica the use of
metaborate will be beneficial.
Silica
is also a well know problem: it is difficult to dissolve in lithium tetraborate
at low temperature of fusion (1000-1100°C).
An
amount of lithium metaborate increasing the basicity of the system and also
decreasing the temperature of fusion, permits better homogeneous dissolution of
the oxides in the melt.
Selection
of fluxes based on laboratory equipment.
In
order to obtain accurate results the sample as to be completely dissolved in the
flux and the dispersion in the melt must be homogeneous.
The
temperature of fusion and an efficient stirring and swirling of the melt during
the fusion are very important factors to obtain good analysis results.
It
is generally accepted that the selected temperature of fusion has to be higher
by 100/150 °C than the fusion temperature of the flux/system of fluxes in order
to obtain a very homogeneous solution.
Heating
systems:
a)
Muffle furnace :
A
good ventilated and isolated muffle furnace regulated and heated at 1100°C with
a good temperature regulation system is a very good and cheap method
b)
Gas burners :
Some
equipment on the market permits the fusion of from 2 up to 6 samples at once.
Sometimes the temperatures of LPG or Propane gas heated burners are not high
enough to bring the melt at 1000-1050°C.
Some
fusion machine are equipped with oxygen injectors to
increase the fusion temperature separately and accurately.
To
avoid differences of heating temperature of each burners which results in severe
loss of reproducibility and accuracy, we recommend the use of fusion
machine equipped with individual burner
regulation.
c)
Induction heating:
The
heating by induction coupled with shaking is of course is to be recommend: the
Philips Perl’X3. The advantage of this equipment is the possibility of
automatisation and a precise regulation of the heating temperature and the
possibility to select various fusion programs. The efficiency of rocking is
vigorous but the swirling is weaker. In this case the use of a flux with a
particle size distribution between 100 and 350 µm could improve the
homogenisation and the perfect dissolution of the sample in the flux by
increasing the surface of contact of sample/flux.
General
recommendation
The
temperature of the melt should not exceed 1100 °C, in some case in order to
avoid the loss of volatile oxides such as Na2O, K2O, SO3 etc. and should be less
than 1250 °C to avoid evaporation of Li2O.
The
addition of lithium metaborate in lithium tetraborate decrease the fusion point
of the melt and these mixture, for instance a FLUX
LT65:LM35 can be used if the heating is insufficient or when
we need to fuse at low temperature to avoid loss of volatile, such as sulphur.
It
is easy to detect if the specimens are not well dissolved by the examination of
the bead.
Bad
dissolution gives crystallisation, cracking of the glass-disk, opaque glass
beads.
Some
specimens are very difficult to dissolve for instance chromite sand. The use of
a mixture of lithium tetraborate, lithium metaborate and lithium metaphosphate
is to be recommended.
Particles
size distribution of the oxidised specimen and the flux
To
increase the specific surface of the system specimen/flux it is necessary to
grind the specimen as fine as possible in general the size should be comprise
between 80 and 200
µm.
We
produce special grades with a lower particles size distribution in order to
obtain a better contact between the sample and the flux and obtain a better
dissolution
Additives
One
of the advantages of the fusion is the possibility to add on a homogeneous way
some additives. These additives are helpful for the analyst. These additives
could be oxidant, anti-wetting agent, internal standard, and heavy
absorbers.
Oxydant:
Specimen
containing non-reduced species such as ferro-alloys has to be oxidised in order
to avoid the destruction of the platinum labware.
Oxidant
is generally nitrates of sodium, potassium, lithium and strontium, perborate.
The preferred oxidant is lithium nitrate because this product does not interfere
with the analysis results. As lithium nitrate is very hygroscopic one could also
used strontium nitrate. Ammonium nitrate decomposes very easily and the
oxidation could be incomplete.
Non-wetting
agent:
A
small quantity of halides reduces the superficial tension of the heated liquid
and the pourability is better. The beads stick less in the casting dish.
The
best wetting agents are respectfully the iodates, iodides, bromides, and
fluorides.
The
amount to be added is very small, 0,1 or 0,2 mg are enough. This quantity has
practically no influence on the analysis results.
Lithium
carbonate:
Some
formulation contains up to 10-20 % lithium carbonate and have the reputation to
dissolve more easily acidic oxide than lithium tetraborate alone and seems to
have oxidising properties. In our opinion we believe that the rate of
dissolution increases because of the formation of lithium metaborate.
The
oxidation power is limited because the lithium carbonate at high temperature
decomposes quickly in lithium oxide and carbon dioxide.
It
helps also to produce bubbles in the melt for a better mixing.
It
also initiates the reaction of decomposition of nitrates and is often mix with
them.
Heavy
absorbers:
Heavy
absorbers are less used today to maximise the sensitivity of the new
spectrometers and because of the availability of great choice of programs on
computers.
FLUX
SPECIFICATIONS
Fluxes
properties
· Fused,
white vitreous powder
· Non
dusty
· Free
flowing
· Non-hygroscopic
· Low
loss of ignition: 0,05% typical
· High
bulk density: 1,2g/cm3 typical
· Perfect
homogeneity
· Controlled
particle size distribution: STD 100 - 600
µm or Grade G 100 - 350µm
· Pure
Level
of Impurities
| 0
- 1 |
ppm |
Pb,
Ni, Mn, Cd, zn, Co, Ag |
| 1
- 5 |
ppm |
K,Cu,
Se, As, Al, Sn, Na, Fe |
| 2
- 10 |
ppm |
Si,
S, Ca, Mg |
Platinum
& Platinum Alloy Labware
Products
We
supply an extensive range of platinum and platinum alloy products.
·
Crucibles
·
Dishes
·
Beakers
·
Fusion labware
·
Boats, tongs, spatulas, electrodes,...
We
also have the capabilty to assist in the design and manufacture of specialised
items.
Service
We
offer a complete service with , refining
and technical support.
Serving the Labs. 
