Dehydration studies of borax penta-and decahydrate minerals were carried out using a microwave (MW) heating system. A 650 W microwave oven (2450 MHz) as well as a 1600 W convective heating unit was used. It was possible to produce 97.5% Na2B4C>7 anhydrous borax products. Preheating affected both the quality and dehydration time of the end products. The dehydration was found to be very fast up to 2 mol of water after which its speed dropped progressively. Overall microwave heating produced cleaner products and was much faster than the conventional heating system.
In this study, drying of borax pentahydrate
[Na2B4O7·5H2O]
(BPH) by microwave energy is
investigated. Using 100−700 W and 2450 MHz microwave energy,
5−10% moisture containing
BPH placed on a Teflon-lined belt is dried at 60−100 °C
selectively, without any loss of water
of crystallization. In this process the microwave oven is first
set to 70 °C and then wet BPH
which is previously heated to 70 °C is placed in it. The sample
first heats up to 80−100 °C by
the effect of microwave energy, and then its temperature sharply drops,
resulting in complete
removal of moisture. This process gives way to no decomposition
and disintegration of hydrated
crystalline molecule and there are no contaminants involved.
Therefore, dry borax pentahydrate
dried this way retains its original purity and physical form.
Since there is no need to heat the
drying system and the air and to keep the relative humidity of the air
constant, microwave
drying is more economical than the conventional process. In
addition, a sharp decrease of
temperature right after complete drying gives way to easy control of
the process by simply
regulating the temperature.
Industrial production of anhydrous borax is via fusion
of borax deca- or pentahydrate at 800−1000 °C. The glassy melt obtained thus is then cooled to
solidify, crushed, ground, and then
sieved to be classified according to its particle size and
distribution. Since molten borax is highly
corrosive to refractories and steel, among all these operations, the
melting of borax is the most
critical and the most costly. Our study gives way to production of
commercial quality anhydrous
and crystalline borax by eliminating melting and other above-mentioned
operations. For this
purpose, borax pentahydrate is dehydrated in a fluidized bed with
gradually increasing the bed
temperature up to 550 °C. During this process, as the bed
temperature is increased gradually,
particular attention is paid to keep the dehydration rate below a
certain value to secure the
bulk density of the product at levels greater than 0.3
g/cm3.
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