Note: Descriptions are shown in the official language in which they were submitted.
s~
Solut]ons o~ alkali silicate, i.e.~ water glass,
or alkali water glass, are usually obtained by fusing virtually
pure quartz sand with soda, i.e., sodium carbonate, or potash at
1400 to 1500C and subsequently dissolving the glass pieces
obtained in water under pressure at elevated temperatures. This
method is very elaborate, both in terms of equipment and energy,
and requires relatively high energy and investment costs.
It is also known to react amorphous residuary silicic
acids with sodium hydroxide solutions to form sodium silicate.
~owever, a disadvantage to this method is that since pure resid-
uary silicic acids are not available in sufficient quantities,
it is usually necessary to use somewhat contaminated raw materials.
For example, the waste gases formed in the production of-silicon
or ferrosilicon alloys in electric furnaces contain solid
components - the so called flue dust ~ which are obtained in
considera~le amounts as waste products in the puri~ication of
these waste gases~ The flue dust consists mostly (from about
89 to 98%) of amorphous silicon oxide, which is contaminated by
other metal oxides and carbon.
A method for the preparation of water glass solutions
by reacting~these flue dusts with alkali metal hydroxide and water
at temperatures between 75 and 100C is described in DOS 2,619,604.
The~water glass solution thus obtained contains, howeve~, impurities which,
depending Qn the raw material used, can be removed by filtering
the water glass solutions through a presssure filter. Unfortunately
the manner in which this filtration is to be effected in practice
_ ] ~
5g~
is not further discussed -ln this patent, and a separation of' the
urities is deliberately fore~one in the examples. Tests have
shol:ln, however~ that the water glass solutions obtained according
o this ~atent are barely filtQrable, (See, for example, Comparison
_xa,lple 1 below.) Thus, only greatly contaminated water glass
is obtained with thls method, and such water glass can be used
on~y for a few special technical purposes.
Applicants have surprisingly discovered a method of
preparing water glass solutions from the amorphous residuary
silicic acid flue dust wherein filterable water glass solutions
are prepared and uncontaminated sodium silicate can be recovered.
suc~ uncontaminated sodium silicate has many technical applications.
.
~ .
It is an object of this invention to provide a method
for preparing water glass solutions from which uncontaminated
wa.,er glass can be recovered.
It is further an object of` this inventlon to provide
~ a method for preparlng water glass solutions from such flue dust
- ~ by T,rhich filterable water glass solutions free of` insoluble
- ~esidues can be obtained.
These and further objects of the invention will be
e~ldent from the discusslons below,
~ . . .
~5~L8
This invention is directed to a method of preparing
water glass solutions frorn which uncontaminated water glass can
be recovered. More particularly~ this i,nvention is directed to
a method for the preparation of water glass solutions by reactin~,
flue dust, obtained in the production of silicon or ferrosilicon
alloys, with aqueous alkali-metal hydroxide solutions at elevated
temperatures, and subsequently filtering the solutions obtained.
According to the method, flue dust is treated in an autoclave
with a ~rom about 6 to 15% by weight aqueous alkali metal hydroxide
solution at temperatures of from about 120 to 190C and a pressure
of from about 2.9 to 18.6 bars, the weight ratio of alkali metal
hydroxide solution to solid flue dust being from about 2:1 to 5:1.
.
It was surprisingly found that reac-tion under the
indicated condit~ons leads to water glass solutions which can be
easily filtered. Thus, water glass free of lnsoluble residues
and useful for many purposes can be obtained in good yields frorn
these flue 'dusts, which heretofore were merely deposited as waste
products.
The term "water glass" is broadly intended to refer to
alkali metal silicates~ such as sodium silicate and potassium silicate.
~ ~ .
It is of great importance that the weight ratio of
the aqueous alkali metal hydroxide solùtion to the solid flue
dust used be at least about 2:1. If this ratio is lower~ a
water glass is not obtained but, rather, a solid reaction product
which can only be removed from the autoclave by mechanical means.
:`
(See Comparison Example 2). Preferably the weight ratio of
alkali metal hydroxide solution to the solid flue dust is in the
:
s~
r~n~e of from about 2:1 to 5:1. When reac~ion mixtures with
~;ei~ht rat~os above 5:1 are used, water glass solutions are also
produced but the sollltions are usually much th:inner and must L)e
concen~rated for further use.
Pre~erably temperatures in the range of ~rom about 120
to 190C are maintained during the reacti.on, s~nce the formation
of water glass solutions is temperature~dependent under the
indicated reactîon conditionsO This dependency co~reeponds to
the desired molax ratlo of SiO2 to Me20 tMe - alkali metal~ in
the final product. I~ the production o~ wa~s glass w~th a higher
- sil~ca content, e.g., with a molar ratio o~ SiO2 to Me20 of about
4:1, is desired, i~ has been found to be advantageous to keep the
reaction temperature in the range of i30 to 170C.
- : . . .
The pressure in the autoclave should be in the range Or
~o~ about 2.9 to 18.6 bars during the reactlon. Higher pressures
can generally be used, but they do not lead to better results and
- only require more elaborate measures. If pressures near the
lo~ r limi~ o~ the preferred pressure range are ko be applied,
~he selected reaction temperature must be given careful considera-
t~on. Xn view of the weight ratio of the alkali metal hydroxide
J
solution to the s~lid flue dust to be maîntained, the pressure
should be nigh enough that the wa~er in the autoclave remain~
in ~he liquid phase at the given tempera~ure. Preferably the
~ressures are in the range of from about 8.8 to 1ll.7 bars.
Bo~h sodium hydroxide and po~assium hydroxide can be
used as alkali metal hydroxides~ The aqueous solutio~s used
should preferably have an alkali me~al hydroxide content in the
ran~e of from about 6 to 15% by weight. ¦~
114~91~
In the practice of the lnventLon, the ~lue dust ernployed
is obtained from the production of sillcon or ferrosilicon alloys
The flue dust usually has the :~ollowing composltion:
_omponent Percent b~ Welght
SiO2 ~9 - 98
SiC 0.2 -- 0~7
C 0.2 - 2.5
23 0-05 -- 2.5
A123 0~1 - 1.5
TiO2 0.01 - 0.05
CaO 0.07 - o.8
rago: 0.2- 1.5
Na20 0.1 ~
K20 0.3 - 2.2
P~ ~ 0.03 - 0.1
~:~ s 0 03 - 0 . 5
The form in which the flue dust is used is not critical.
It can be used in powder form or in the form of granules or
pelleks.
The water glass solutions obtained in the reaction are
readily filterable by use of conventional filtration techniques,
such as, for example, filtration through a porcelain funnel with
Perlo ~ filter or, in the case of more viscous liquids, by means
of a presssure fi]ter. The fiItrate represents a residue-free,
mostly clear~ yellow water glass composition having a solid5
content of from about 15 to 35% by weight~ depending on khe flue
~ dust and the alkali metal hydroxide solution used. Preferably
;~ the molar ratlo of SiO2 to Me20 ranges from about 2. 2 1 to l~
.~ :
,
~ L59~
~ ater glass, i.e.~ an alka1ilnetal silicate such as sodium
or potassium si:Licate, can be recovered from the water glass
solution by using known techniques, such as evaporation.
The water glass obtained according to this invention
can be used ror various known purposes. For exampleg the water
gIass can be used as an additive in detergents or cleansers~ as
binder or adhesive for coatings and impregnations, for the pro-
duction of fillers, for the preparation of inorganic silicates
or silica gels, and the like.
Depending on the intended use of the water glass,
decoloration of the slightly colored solutions may be desirable.
; This can be easily achleved, for example, by the addit~on of
; active carbon and subsequent filtration. DecoloratiQn of the
water glass solutions is also possible by the use of oxidants,
e.g., 30% hydrogen peroxide solution. Water-clear water glass
solutions can also be obtained if the flue dust ls treaked with
atmospheric oxygen at temperatures of 400 to 600C prior to
reaction with alkall metal hydroxide soluti.on.
As mentioned above, the method according to the invention
20 generally leads to a high yield of silicon dioxide~ related to
the flue dust used, depending on the reaction conditlons. High
yields can be obtained particularly by additional processing of
the filter cake remaining after filtration. Due to their
porous structure andlarge surface area~ filter cakes still con-
tain large quantities of water glass, which can be washed outwith water or with the corresponding diluted alka]i metal hydroxide
solution to yield diluted water glass solutions. In a preferred
embodiment of the invention, the diluted water glass solutions
obtained by washing the filtercakes, are admixed with fresh
: ~ :
-6--
~5~8
alkali metal hydroxide solution and recycled to be reacted with
additional flue dust.
E X A M P L E S
The following exa~.ples are set forth to demonstrate
certain embodiments of the invention herein and are not to be
construed as limiting the invention hereto.
Example 1
A reaction mixture comprised of 301 g o~ a slurry of
granulated flue dust (water content 25%; SiO2 content in the
flue dust itself, about 90%), 192 g of 50% aqueous sodium hydro-
xide solution and 539 g of water, was charged into a conventional
stirring autoclave of chrome-nickel steel with a capacity of
two liters. Thus~ the reaction mixture comprised 226 g of flue
dust and 806 g of aqueous sodium hydroxide solution with a content
of 118.7 g ~aOH/1, the rakio of sslution to solids being 3.57:1.
The autoclave was heated and, when the heating was starte~d, an
lnitial pressure of 3.9 bars was generated in the autoclave by
use o~ an inert gas. ~'he reaction mixture was then heated to
170C and stirred for two hours at a pressure of 14.7 bars.
Subsequently the reaction mixture was filtered without any
difficulty by means of a Perlo ~ filter over porcelaln funnel.
The filtrate obtained was comprised of 429 g of water glass
solution having a solids content of 26.9% and a SiO2 : Na20
ratio of 2.5:1 (19.23% Si02, 7.69% Na20).
The remaining filter cake was subsequently washed out
twice with distilled water. In the first washwater, 600 g of
water glass solution (8.84% SiO2~ 3.59% Na20) were obtained,
.
-7- ~
ani, in t'ne second washwater, 465 g of water glass solution
.3~% sio2, 2.15% Na20) were obtained.
The total yield of~ Si02 as a dissolved water glass
c~.?onent was then 79%.
Unless otherwise noted, the following examples were
car~ied out according to the procedure of Example 1.
le 2
. Reaction Mixture:
226 g of flue dust (about 90% Si02)
806 g of sodium hydro~ide solution with a content
of 118.7 g NaOH/l ( 192 g of' 50% sodium
hydroxide solution and 614 g of water)
Ratio of solution to solids: 3.57 :1.
The reaction mixture was heated to 170C and stirred
15 for one nour at 14.7 bars. The reaction mixture was filtered,
snd the filtrate was found to be comprlsed of 560 g of water
gl3.ss solution with a solids content of 27. 5% and a Si02/Na20
ra~io of 2.64:1 (19.96% sio2~ 7.55% Na20). ~he filter cake was
~:Jashed twice, the f'irst washwater yielding 502 g of' water glass
so-ution (9.21~ Si02, 4.05% Na20) and the second washwater
yielding 435 g of water glass solution (2.9% sio2, 1.2% Na20).
The total yield of S102 was 84L.
Exam le 3
Reaction Mixture:
225 g of flue dust (about 90% Si02)
808 g of sodium hydroxide solution with a content
: of 90.3 g NaOH/l (149 g of 50% sodium
hydroxide solution and 659 ~ of' water)
Ratio of solution to solids: 3.59:1.
.
~ -8-
~S~8
The reaction m.i.xture was heaked to 150C and ~tirred
for two hours at 8.8 bars. The reackion mlxture was ~ilkered,
and the filtrate was fOUIld to be compri.sed of 618 g of water g~a~s
solution with a solids content o~ 2307% and a SiOz~Na20 ra~io
of 3.. 33~ .2% SiO2; 5.46% Na20)~ The fil.ter cake was washed
t~ e~ the ~irst washwater yielding 267 g of water glass
solution (11.81.% SiO2, 3.7% Na20), and the second washwater
yielding 344 g of water glass solutiorl (4.94% SiO2g 1.65% Na~O).
The total yield of SiO2 was 79%.
Example_4
.
. Reaction Mixture:
_ .. . :
300 g of flue dust (about 90% SiO2)
777 g of sodium hydroxide solukion with a content
- of 96.~% NaoH/l (150 g of
50% sodium hydroxide solukion and 627 g o~ -
water) -
Ratio o~ solution to solids: 2.59:1.
The reaction mixture was heated to i30C and stirred
for one hour at 8.8 bars. The reackion mixture was filtered,
and the filtrake was found to be comprised of 510 & of water
glass solukion having a solids. content of' 26.3% and a SiO2~Na20
ratio of 3.~5:1 (20~91% SiO2; 5.43~ Na20). The filter cake was.
washed twice, the first washwater yielding 445g`o~ wa~er glass
. ~olution (11.62% SiO2;.3.18% Na20), the second washwater yield-
ing 435 g of water glass solution (Il.27% SiO2, 1.36% ~a20).
: The total yleld of SiO2 was 64%.
:
Example 5
.
: Reaction Mixture~
~ . i
196 g Or flue dust (abouk 90% SiO2~
846 g o~ sodium hydroxide solution with a content
of 70.1 g of ;~aOH/l (119 ~ of 50~ sodium
hydroxide solution and 727 g of water).
Rakio of solution to solids: I~.32:1.
_9_ .
~s~
The reacti.on mixture was heated at 150C ~or two hours
at ~.8 bars and then filtered to obtain a filtrate comprlsed of
73 g of l.rater glass solution having a solids content of ]9.1%
and a SiO2/Na20 ratio of 3.18~ 4 5jt SiO2; 4.56% Na20). The
filter cake was washed twice to obtain, in the first washwater,
37l~ G of water glass solution (5 63% SiO2; 1.91% Na20) and, in
the second washwater, 249 g of water glass solution (2.79% SiO2,
1.05~o Na20). The total yield of SiO2 was 76%.
EY.am~le 5
Reaction Mixture:
180 g of flue dust (about 90% SiO2) .
846 g of sodium hydroxide solution with a content
of 70.1 g of NaOH/1 (119 g of 50% sodium
hydroxide solution and 727 g of water)
Ratio of solutlon to solids: 4.7:1
The reaction mixture was heated in the autoclave at
150C for 30 minutes at 8.8 bars. The reaction mixture was
fi~t~red, and the filtrate was found to be comprised Or 750 g o~
water glass solution having a solids content of 18.3% and a
Si~2/Na20 ratio of 3.02:1 (13.75% SiO2, 4.55% Na20). The filter--
cake was washed with distilled water twice, the first washwater
comprising 339 g of waterglass solution (5.54% SiO2; 2.03% Na20),:
and the second washwater comp.rising 308 g of water glass solution -
~2.11% SiO2; 0.81% Na20). The total yield of SiO2 was 79,0.
:
Exam~le 7
Reaction Mixture:
346 g of flue dust (about 90% SiO2)
728 g of sodium hydroxide solution with a content
of 13~.4 g of NaOEI/l (204 g of 50~ sodium
hydroxide solution and 52ll g of water)
Ratio of solution to solids: 2.1:1
-10--
The reaction mixture was heated in an autoc:Lave at
150C for t~o hours at 8.8 hars. After ~he reackion mlx~,ure had
cooled, a highly viscous liquid was obtained. This llqu:;d ~ras
filtered by means of a pressure ~`i.lter at 90C and 2.9 ~rs ~o
produce 334 g of water glass solution having a sol:lds c(~ntent
of 33.6% and a SiQ2/Na20 ratio of 3.38-1 (25.94% S:102; 7.67%
Na20). The ~iltercake was washed kwice with distilled ~later~
the first washwater being comprised o~ 772 g of waterglass --
(13.84% SiO2; 4.35% Na20) and ~he second washwater bein~ com~sec~
~0 of 560 g of waterglass(4.07% SiO2~ 1.44% Na20~. The total yield
of SiO2 was 69%.
. .
Example 8
Production of potash water glass~ i.e.~ potassium
~ilicake. -.
. .
Reaction Mixture . .
280 g of:~lue dus~ (about 90% SiO2~
806 g of ~otassium hydroxide solution with a
content of 120 g KOH/1 (114 g o~ 85
. potassium hydroxide solution and 692 g
water).
Rakio of solution to solids: 2.88:1
The react..on mixture was heated, wikh stirring~ at
:: 170C for kwo hours at 14.7 bars. The reaction mixture was
filtered, and the filtrate was found to comprise 804 g of potash
. . .
~!ater glass ~'~lut`ion~ having a solids content of 27.1% and a
SiO2/K20 ratio o~ 2.41:1 (19.14% SiO2, 7.94% K20), The filtercake
was washed with diskilled water twice, the first washwaker
comprising 549 g of pokash water glass solution (4.o6~ SiO2; 2.04%
K20) and the second washwater comprisin~ 367 g of pokash wat.er
3o ~lass solution (1.l~5% SiO2; 0.93% K20). The total yield of
SiO2 was 72%-
.
~ s~
. C~ r~so~ E~amole 1
_ . . . - , ;
In accordance ~tith the data set forth in Rxarnple 2 o~
G~rm2n ~u~lished appli.cation (DO~ 2,61~0l~, rlue dust ~as reacted
w~ a~ueous sodium hydroxide wlthou~ the a~plication o~ pressvre.
Reactlcn Mixtu-re:
367 g of ~lue dust (about 90% SiO
848 g of sodium hydroxide solution with a ~onten~
. of 81 g of NaOH/l (138g of 50% sodium
hydroxide solution and 710 g o~ water)
~: .
lQ The react~on mixture was heated in an open vessel und~
s~i~in~ to 85C for 30 minutes and subsequently transferred t^
.~no4her cold vessel ~or c~oling. The cooled reaction mixture
cc~?rised 1,210 g of wat~r glas~ solution, which could not be.
~lt~red, however, either over a porcelain ~unnel wlth varlous
~ 15 t~p~s ~ lilters (Perlo ~ paper, etc.).or over a pressure ~ilter
: (2.~ bar)~
' :, ' ,' ', ',
C~-~arison Exam~le 2
Reaction M~xture:
.
500 g of flue dust (about 90~ SiO2) ..
~0 600 g o~ sodium hydroxide solution with a conten~
of 148.4 g NaOH/l (178 g o~ 50% sodlum
hydroxide solution and l~22 g of ~ater)
Ratio of solution to solids: -1.2:1
The reaction~m~xture was heated to 150C at 8.8 bars.
~: 2~ ~o..eJer, af~er the desired reaction temperature o~ 150~C had been
;~ ~ ~t'~ained, the reaction mixture became solid and could no longer :
~:~ o~ s~irred. Lia,uid wa~er glass could not be ob~ained this ~ray.
,
, ' '
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