Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR THE PREPARATION OF ALKALI METAL
OR ALRA~IYE L~ AL PEROXIDES
~OI/ND 0~ ~r Ill~
The present invention pertains to a process for
the preparation of alkali metal or alkaline earth metal
peroxides.
Alkali metal and alkaline earth metal peroxides
are increasingly used in industry. Thus, for example 7
alkaline earth metal peroxides are used for medical and
pharmacological purposes. Recently~ CaO2 has been gaining
in importance because it serves to improve the growth of
culti~ated plants~ The slow release of oxygen by peroxide '
compounds is utili~ed in s~wage treatment technology.
Alkali metal and alkaline earth metal peroxides
ar~ prepared, in general, by producing peroxide-containing
reaction mixtures from aqueous solutions of their salts,
oxides or hydroxides by means of conversion with an aqueous ¦
hydrogen peroxide solu~ion~ centrifuging or filtering the
reaction mixtures and drying them on tray driers. The
dry material is convent~onally ground and optionally
screened~
DE-OS No. 15 42 642 discloses a process for the
pxoduction of CaO2 wherein a highly diluted solution of
hydrogen peroxide is reacted with an excess of calcium -`
hydroxide at temperatures under 30C to form calcium
peroxideoctahydrate, which is ~hen converted in a sub-
sequent drying step into anhydrous peroxide.
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All of these processes are affected by a series
of disadvantages. For example~ they are expensive because
it is necessary to separate the product from the mother
li~uor by centrifuging and then to grind the tray-dried
product. Furthermore, the economy of the processes is
achievable only to a limited extent, because appreciable
losses of peroxide are caused by the decomposition of
hydrogen peroxide in the large volumes of liquor resulting
from the relatively dilute suspensions and by the long
retention time in the drying procedure.
In addition, large volumes of water must be
unnecessarily transported because of J he high dilution of
the reaction mixture and then removed in an energy-inten-
sive drying stage~ Supplemental control devices to control
the temperature and additional process steps to eliminate
the excess calcium hydroxide are also required. A further
disadvantage resides in a certain inhomogeneity of the
final product, caused by decomposition during drying and
during the subsequent grinding process.
2 0 SUMMARY OF THE INVENTION
,
It is therefore an object of the present invention
to provide an improvea process for the preparation of an
alkali metal or alkaline earth metal peroxide.
Another object of the present invention resides
in providing a process for the preparation of an alkali
metal or alkaline earth metal peroxide which is economical
to operate and consumes lower amounts of energy.
A further object of the invention i5 to provide
such a process in which only small losses in peroxide
occur during processing,
Still another object of the invention resides in
providing a process which produces an improved peroxide
product and in providing such an improved product.
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In accomplishing the foregoing objects, there
has been provided in accordance with the present invention
a process for the preparation of an alkali metal or alkaline
earth metal peroxide, comprising the steps of mixing a
concentrated aqueous suspension comprising an alkali metal
oxide or hydroxide or an alkaline earth metal oxide or
hydroxide with an a~ueous solution con~aining hydrogen
peroxide, to obtain a peroxide suspension; agitating
the peroxide suspension for a period of time sufficient
to substantially complete the reaction between the hydrogen
peroxide and the oxide or hydroxide, and subjecting the
substantially completed reaction mixture to spray drying.
Optionally, the process further comprises ~he steps o~
recycling a portion of the reaction mixture to the mixing
step, and/or removing part of the water content from th~
reaction mixtllre prior to the spray drying step, and/or
cooling the mixture of oxide or hydroxide and hydrogen
peroxide after the mixing step and/or concurrently with
said agitating step.
In accoxdance with another object of the invention,
there has been provided an improved alkali metal or alkaline
earth metal peroxide produced in accordance with the pro-
cess defined above.
Further objects, features and advantages of the
present invention will become apparent to a person of
ordinary skill in the art from the detailed description
of preferred embodiments which follows.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
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According to the invention, a concentrated
aqueous alkali metal or alkaline earth me~al oxide or
hydroxide suspension is mixed, preferably accompanied by
cooling, with an aqueous hydrogen peroxide solution, and
the peroxide suspension obtained, preferably with further
cooling and with agitation up until the completion of the
reaction, i5 subjected to spray drying.
Surprisingly, it has been discovered that the
disadvantages of the known processes may be overcome by
working with suspensions which are concentrated as highly
as possible and by drying thes~ suspensions directly and
carefully. The oxide or hydroxide suspension to be used
should have a high solids content. Optimally, a suspension
just capable of being pumped is used. The solids content
of the suspension is determined by the oxide or hydroxide
to be utilized, but normally is higher than about 200 g/l,
preferably higher than about 300 g/l.
The suspension to be used may be prepared, for
2~ example, by the simultaneous metered addition of water
and a commercially available oxide or hydroxide powder
wi~h agitation in a mixing vessel. In a pref~rred embodi-
ment of the process, the suspension which is internally
heated by the exothermic solution process is mixed directly,
without any interim cooling, with hydrogen peroxide.
Commercially avail~ble products may be used as
the hydrogen peroxide solution; it may also contain one
or more known active oxygen stabilizers. It is preferred
to use a H2O2 concentration of higher than 50~ by weight,
most preferably higher than 65% by weight, so that the re-
action mixture will not be excessively diluted.
The process according to the invention may be
carried out as follows:
The corresponding oxide or hydroxide is sus-
pended in a volume of water which is as small as possible,optionally with cooling. The oxide or hydroxide suspension
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obtained is mixed with an aqueous solution of hydrogen
peroxide in an intensive mixing apparatus~ The peroxide
suspension formed is subsequently treated -- optionally
with further cooling -- to the completion of the conversion
in a kneader, and the paste is then pumped to a spray-
drier~ wherein the product is atomized via a rotating
disk in a ~low of warm air.
As the intensive mixing apparatus there may be
used, for.ex~mple, a rapidly rotating excentric pump or
a plough share mixer, optionally equipped with a cooling
jacket. It i5 of advan~age to remove a portion of the
water content of the reaction mixture after combining
and intermixing of the reaction components, preferably,
to a content of approximately 40~ by weight t with re-
spect to the reaction mixture. For the treatment of thereaction mixture up until the point of complete conversion,
there may be used, for example, a conical screw mixer.
In this mixer, the reaction mixture may be kneaded for a
period of from about 0 to 3 hours, preferably from about
0 to 0.5 hours, i.e., to the end of the reaction or to
the optimum degree of conversion. In the process according
to the invention, the paste may be transported by means
of a screw pump. For drying ~he product of the invention,
an atomi7.ing dxyer with a rapidly rotating disk has been
found appropriate. Other types of conveying and spray-
drying equipment can also be used.
The following advantages distinguish the process
according to the invention from the state of the art:
a) by means of the stoichiometric charging of
the reactants and operation with concentrated reaction
mixtures, the proportion of the ballast (water, excess
reactant) is held to a minimum;
b) expensive separation and purification stages
are eliminated by the direct spray drying of the reaction
mixture. Furthermore, the product.is obtained in the
form of a freely flowing granulate and does not require
final treatment in auxiliary comminuting installations;
and
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c) the process may be operated both discon-
tinously or continuously.
The product obtained by the process of the in-
vention is distinguished by the following advantages
from the state of the art:
a) the avoidance of local decomposition re-
actions insures good homogeneity of the product;
b) spray-drying provides good flowability,
uniform crystal structure and a narrow, uniform grain
size range; and
c) because of the intensive drying, peroxides
free of water of cxystallization are obtained.
The process is schematically illustrated by
the diagram in Figure 1. The reference numeral 1 re-
presents the mixing apparatus for the preparation ofthe oxide or hydroxide suspension, while the H2O2 solu-
tion is held in readiness in vessel 2O The charge
solutions are combined in the intensive mixing apparatus 3
and transported by way of the cooler 4 to the conical screw
mixer 5. The retention time in mixer 5 may amount from
about 0 to 3 hours, preferably from about O to 0.5 hours,
depending on process design. By means of a screw pump 6,
the reaction product is withdrawn continuously from mixer 5
and transported to the spray-dryer 7 for spray-drying~ In
a special alternate embodiment of the process according to
the invention, it is possible to remove part of the re-
action mixture by means of a bypass 8 prior to its entry
into the cooler 4 and to recycle it for an accelerated
initiation of the reaction to the mixing apparatus 3.
The mixing apparatus 1 and 3 may be equipped additionally
with cooling devices.
The process according to the invention will be
explained but not restricted by the examples which follow
hereinafter.
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EXAMPLE 1
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In a mixing vessel (l) a Ca(OH)2 suspension is
prepared by means of the simultaneous metered addition
of water and Ca(OH)2 powder under agitation; the suspension
contains 50% by weight Ca(OH)2. In a storage vessel, a
70% by weight aqueous H2O2 solution is held in readiness;
the solution is stabilized with 300 mg PO4 /l. With a
screw pump, 660 kg/h of the Ca(OH)2 suspension from (l)
and with a metering piston pump 186 l/h of the H2O2 - -
solution from (2) are passed reactingly through a rapidly
rotating mixing pump (Supraton 2.07) (3). In this manner,
900 kg/h of a CaO2 slurry are formed with the simultaneous
release of heat.
The slurry is cooled in a thin film heat ex-
changer (4) -- a vertically arranged water-cooled double
cylinder, wherein the slurry to be cooled is thrown by
means of agitators onto the cooling surfaces where it is
cooled and then flow3in the downward direction -- to
308K; it then flows freely into a screw mi~er (5)O
From the screw mixer (5), the CaO2 slurry is
fed by means of a screw pump (6) to a spray~dryer ~7)
for dryingO The dryiny temperatures are set for the con-
tinuous flow of the product and equal to 473K at the
inlet of the dryer and 348~K at its outlet. The product
obtained in this manner (324 kg/h) contains 76.4% by
weight CaO2. The scanning electron microscope image
(Figure 2) shows that the average particle diameter is
around about 5 ~ and that approximately 90~ of the
particles have diameters ketween about 2 and 7 ~.
EXAMPLE 2
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In a manner similar to the method described in
Example l, 496 kg/h of a 25% by weight MgO suspension and
60 l/h of a 70~ by weight H~O2 solution are reactingly
processed.
Subsequently to the mixing pump a partial flow
(30~ by volume) is recycled by way of a bypass (8) into
the mixer pump to accelerate the onset of the reaction;
the Mgo2 slurry (573 kg/h) is cooled -- as described in
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Example 1 -- intermixed in a screw mixer and dried in an
atomizing drier.
The product obtained in this manner (122 g/h)
contains 45~8% by weight MgO2.
EXAMPLE 3
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In a manner similar to the operating mode in
Example 1, 132 kg/h of a 41% by weight LioH suspension
and 47 l/h of a 70~ by weight H20~ solution are processed
reactingly. 193 kg/h of a Li~02 slurry are formed, with
the release of heat.
Processing according to Example 1 yields a
product (52 kg/h) containing 89% Li202.
EXAMPLE 4
In a variant o the process, the CaO2 slurry pre
pared according to Example 1 is fed from the mixing pump
into a decanter centrifuge, where it is dehydrated to a
total water content of approximately 40% by weight. From
there, it is fed into a screw pump, which transports the
concentrated slurry to a spray-dryer with an atomizing
disk. In the directly heated atomizer, the slurry is
dried by the heated air, wh~rein the temperature of the
incoming air is approximately 773K and that of the out-
going air is approximately 383K.
The dry, free-10wing product in the form of a
powder leaving the dryer contains 75% by weight CaO2 and
is practically free of water of crystallization. It may
be directly fed to a storage silo or packaged after
cooling, without grinding~
It should be emphasized that,in spite of the
use of industrial grades of Ca(OH)2, products having good
stability are obtained; hey may be stored undPr normal
storage conditions for months without substantial loss
of active oxygen.
