Note: Descriptions are shown in the official language in which they were submitted.
SK-5~
534
A Method For Preparing Compositions WhiCh Can se Used In
The Production Of Sodium Dithionite; And The Use Of Such
Compositons
The present invention relates to a method of preparing
compositions which can be used in the production of sodium
dithionite; and the use of such compositions.
According to one known method for preparing sodium
dithionite, 40-50 percent by weight aqueous solution of
sodium hydroxide, liquid sulphur dioxide and water are mixed
with the reducingagent mixture comprising 12 percent by weight
sodium borohydride, 40 percent by weight sodium hydroxide and
48 percent by weight water in a conduit system, in which the
mixture is circulated in order to conduct away the high
quantity of heat of reaction generated.Because of the number
of components to be mixed together, it is necessary to make
several adjustments.In addition, liquid sulphur dioxide is
difficult to handle.
Consequently an object of the present invention is to
proYide a method for producing compositions which can be used
in the production of sodium dithionite and which facilitate the
work of the manufacturer of the dithionite, by reducing the
number of components which need be handled, and therewith
- 20 reducing the number of adjustments which need be made, and by
reducing cooling requirements.
This is achieved in accordance with the present invention
by means of a method which comprises wringing sulphur dioxide
into contact with.a recirculating aqueous solution of sodium
hydroxide and previously absorbed sulphur dioxide to a ratio
between sulphur dioxide and the sum of sulphur dioxide and
sodium oxide(SO2~(S02 Na2o)) of 0.61-0.66,preferably 0.63-
0.65, and subsequently pressurizing the resultant mixture
and introducing liquid sulphur dioxide thereinto to a ratio
between sulphur dioxide and the sum of sulphur dioxide and
I,
.
~L~41~34
sodium oxide of 0.75-0.81, preferably about 0.78, and
adjusting the water content to a concentration of sulphur
dioxide + sodium oxide of 10-13 percent by weight, preferably
about 12 percent by weight.
In the first part stage of the method according to the
invention, in which sulphur dioxide is brought into contact
with a recirculating aqueous solution of sodium hydroxide and
previously absorbed sulphur dioxide, sulphur dioxide is
preferably introduced in the form of a preferably dry sulphur
dioxide-containing flue gas obtained in the manufacture of
sulphuric acid.Flue gases obtained from plants which combust
sulphur-containing fuels can also be used, provided that
measures are taken to prevent condensation of the water when
handling the gas with the recirculating aqueous solution of
r 15 sodium hydroxide and sulphur dioxide.The sulphur dioxide
containing gas is preferably brought into contact with the
aforesaid recirculating aqueous solution in a conventional
scrubber plant.
This part method stage is preferably carried out cont-
inuously at approximately atmospheric pressure, a part of thesolution being removed from circulation and replaced with an
aqueous solution of sodium hydroxide, although the part stage
can also be carried out intermittently.The method is controlled
in both alternatives in a manner such that the resultant
mixture exhibits a ratio between sulphur dioxide and the sum
of sulphur dioxide and sodium oxide of 0.61-0.66, preferably
0.63-Q.65.
In the second part method stage, the mixture obtained
in the first part stage is tranferred to a pressurized space
or chamber, for example a space standing under a pressure in
the order of 2.5 bars or more, preferably in the form of a
section or part of a recirculation loop defined upstream by
a recirculation pump and downstream by a constriction, in
which space liquid sulphur dioxide is introduced into the
mixture to a ratio S02/(SO2 + Na2O) of 0.75-0.81, ?refera~ly
-I about 0.78, and the water content being adjusted to a concen-
tration of sulphur dioxide + sodium oxide of 10-13 percent by
wieght, preferably about 12 percent by
. .
. . ,
3 ~L2~.S3~L
weight,so as to obtain aproduct in the form of a solution
which can be stored at atmospheric pressure.
The invention also relates to the use of a composition
prepared in accordance with the method according to the
invention for preparing dithionite by reduction.In use the
composition is mixed with a reducing agent mixture of preferably
the aforementioned conventional kind and is optionally recir-
culated in a conduit system, optionally while diluting the
mixture by adding ~ater.If desired,the mixture can be diluted
with water prior to circulation, instead of during circulation,
or the end product itself can be diluted.
The invention will now be described in more detail with
reference to a plant illustrated schematically in the accom-
panying drawing and intended for the preparation of sodium
dithionite in accordance with the invention.
In the drawing the references 1 and 2 identify respectively
a road tanker and a tank trailer.The tank of the road tanker
1 is adapted to contain liquid sulphur dioxide and the tank
trailer 2 to accommodate a sodium sulphite-bisulphite solution
20 in a ratio SO2~(SO2 + Na2Ol in the order of 0.61-0.65 and a
concentration in the order of, for example, 40 percent by
weight.The total concentration of sulphur dioxide + sodium
oxide in the combined contents of the tanks of the tanker 1
and the tank trailer 2 is suitably about 50 percent by weight.
The reference 3 identifies a tank having an internal
prevailing pressure of approximately atmospheric pressure and
forming part of a recirculation system comprising a circulation
pump 4, a conduit 5 extending from the tank 3 to the suction
side of the pump 4, and a conduit 6 which extends between the
3Q the pressure side of the pump 4 and the tank 3 and which
incorporates a closure YalYe 7, a restriction 8, and a conduit
part 4 of enlarged crossection.The tank of the road tanker 1
can be connected to the conduit part 9 by means of a conduit
l incorporating a pump 10, while the trailer tank 2 can be
connected to the conduit 5 by means of a conduit ~3 incorpo-
rating a pump 12.The refernce ~4 identifies a water-supply
conduit which branches into three conduits 15,16,17, of which
the conduit 15 supplies water to the conduit 5 for diluting
a sulphite-bisulphite solution conveyed through the conduit 13.
4 ~2~534
The flows through the inlet conduits 11,13,14 and in the
recirculation systems 3-9 are adjusted with the aid of control
means not shown, and by adjusting the constriction 8, so as to
maintain in the conduit part9 a pressure in the order of, for
example, 2.5 bars while emptying the road tanker and the tank
trailer 2.This avoids in a favourable manner the need of
providing conventional, expensive pressure vessels.With the
aid of the hitherto described arrangement there is prepared
and supplied to the tank 3 a sodium sulphite-bisulphite solution
having a ratio SO2/(SO2 + Na2O) of 0.75-0.81, and a concent-
ration (10-13 percent by weight) which enables it to be
stored at atmospheric pressure.
The pressure side of the pump 4 can be connected to a
storage tank 20 by means of a conduit 19 provided with a closure
valve 18, the pressure prevailing in the storage tank also
being approximately atmospheric pressure.Gas departing from
the upper part of the tank 20 is led-off subsequent to being
washed in a scrubber 21 or the like with water supplied
throughthe conduit 17.This water, with sulphur dioxide dis-
solved therein, is supplied to the tank 3 through a conduit
22.Gas collected in the upper part of the tank 3 is sprayed
with water passed through the conduit 16, while undissolved
gases are conducted to the upper part of the tank 20 through
a gas conduit 23.
For the purpose of preparing sodium dithionite, solution
from the tank 2 is combined with a conventional reducing~aqe~t
comprising substantially an aqueous solution of sodium boro-
hydride and sodium hydroxide.This is effected in a recirculation
circuit incorporating a circulation pump 24, a vessel 25, a
conduit-26 extending between the bottom of the vessel 25 and
the suction side of the pump 24, and a conduit 27 which extends
between the pressure side of the pump 24 and the vessel 25 and
in which there is arrranged a heat exchanger 28 for cooling
the circulating liquid to a desired temperature level.The
sodium sulphite-bisulphite solution is supplied to the conduit
27 through a conduit 30 which incorporates a pump 29, while
the solution of reducing agent is supplied to the conduit 27
from a storage tank 3~ through a conduit 33 incorporating a
pump 32.Prepared sodium dithionite solution departs through a
5 ~4~J3~
spillway 34 arranged in the vessel 25 and is conducted to a
storage tank 35 through a conduit 36, optionally after being
diluted with water entering through a conduit 37.Prepared
dithionite solution is conveyed to consumer sites from the
tank 35 through a conduit 39 incorporating a pump 38.
In order to avoid yield losses due to hydrogen formation
in the recirculation circuit 24-28, it has been found highly
suitable to charge the solution of the reducing agent, and
advantageously also the sulphite-bisulphite solution, to the
liquid circulating in the circuit 24-28, substantially iso-
kinetically, i.e. the supplied solute s are introduced into
the conduit 27 in a direction and at a flow rate which coin-
cides substantially with the flow direction and flow rate of
the recirculated solution.To this end, the conduits 30 and 33
discharge into the conduit 27 through nozzles 41 directed
downstream, as eYident from the part 40 of said conduit 27
shown in axial section,wherewith each conduit 30 or 33 can be
branched in the manner indicated, each branch being provided
with a nozzle 41 and an adjustable valve 42.The solution
20 leaving the circulation systems 24-28 through the conduit 36
can be admixed, for example, with a complexing agent for
separating undesirable metal contaminants, and also with pH-
regulating additives.
Example
Sulphite-bisulphite solution obtained from a flue gas scrubber
adapted for sulphur-dioxide containing flue gas derived from
a sulphuric acid plant hazing a sulphite bisulphite concen-
tration in percent Na2O + SO2 = 39 and a ratio SO2~(SO2 +
Na2OI of 0.63-0.65 was used together with liquid sulphur
dioxide and water to produce a composition suitable for the
preparation of dithionite.
In this respect 4680 kg of liquid sulphur dioxide were
charged to a pressurized space and added with 22,000 kg 39 %-
sulphite-bisulphite solution,having a ratio SO~(SO2 + Na2O)
35 of Q.64,and 7q,320 kg water, which gaYe SO2~(SO2 + Na2O) =
0.775 and a concentration of ~2 %.
The solution was passed to a storage tank in which a
pressure equal to atmospheric pressure was maintained and
then used directly for preparing dithionite, it being necessary
6 ~L24~534
only to add a reducing agent mixture,for example a reducing
agent retailed by Ventron Corporation, Mass. USA; under the
registered trade mark BOROL (R) containing approximately 12
percent by weight NaBH4 , 40 percent by weight NaOH and 48
percent by weight H2O.The outgoing dithionite solution was
diluted with water to a concentration of 3 % by weight.
Example 2
Example 1 was repeated but with the ratio SO2~(SO2 + Na2o)
of the sulphite-bisulphite solution equal to 0.65; and this
time 4333 kg of liquid sulphur dioxide and 76,775 kg water
were added, so as to obtain the same factor of 0.775.
Example 3
Example 1 was repeated and the same sulphite-bisulphite
solution as that in Example 1 was used, although with the
desire to obtain a product having a ratio of SO2~(SO2 + Na2O~
= 0.790 kg.CcnseqUently 5741 kg of liquid sulphur dioxide
and 85,684 kg water were added to the 20,000 kg sulphite-
bisulphite solution in this example.
