Note: Descriptions are shown in the official language in which they were submitted.
6~C~
BATTERY SEPARATOR
Description of the Inventlon
The present invention is dlrec~ed to a siliceous filler-contain-
lng battery separator. In commonly used electrlc storage batteries, such
as the well-known 12-volt battery employed in automobiles, separators are
placed between bat~ery plates of opposite polarity to prevent the two
plates from touching each other and causing an electrical short. The
separator is typically a microporous article fabrlcated from a polymeric
material, e.g., natural or synthetic rubber, or a polyolefin. The separa-
tor may ha~e a backing material of, for example, a non-woven web. The
pore size of the microporous separator should be as small as possible
since this reduces the danger of active materials being forced through or
growing through the separator, thereby causing an electrical short.
The separator should also have a low electrical resistance in
order to maximize the power output from the battery. Lower electrical
resistance can be obtained by reducing the overall thickness of the sepa
rator, e.g., the thickness of the backing material; however~ thinner sepa-
rators are more subject to corrosion and other physical factors affecting
the service life of the separator.
Certain siliceous fillers have been used to prepare mlcroporous
battery separators. See, for example, U.S. Patent 2,302,832, which
describes the use of a silica hydrogel in a rubber binder, U.S. Patent
3,351,495, which describes synthetic and natural zeolites, precipltated
metal silicates, such ~s calcium silicate, and silica gels as the inor-
ganic filler and extender for separators of high molecular weight poly-
olefins, and U.S. Patents 3,696,061, 4,226,926, and 49237,083, which
describe the use of finely di~ided, precipitated amorphous s-llica, such
as E1i-Sil~ 233 siliceous pigment, in microporous battery separators.
Typlcally, amorphous precipitated slllca plgment is used to
introduce porosity into the polymerlc material utilized to form the bat-
tery separator. This siliceous pigment is highly absorbant and can
absorb a substantial quantity of an aqueous or organlc liquid-whlle
remaining free flowing. In practice, the amorphous precipitated sillca
is loaded with the liquid and then blended with the polymeric material.
The liquid absorbed by the silica filler is subsequently removed to
impart porosity to the polymer.
It has now been discovered that certaln precipitated siliceous
fillers permit the fabrication of battery separators having reduced elec- -
trical resistance compared to separators prepared with conventional amor-
phous precipltated silica pigments, such as the aforementioned Hi-Sil~
233 silica pigment. In accordance witb the present invention9 a sili-
ceous pig~ent composed of agglomerates of essentially hollow spherical
precipitated siliceous partlcles is used to prepare microporous polymeric
battery separators. Thls siliceous pigment is generally characterized by
a surface area of between about 50 and 250 square meters per gram, a pre-
dominant hollow spherical unit particle size of between 0O005 and 5.0
microns and an oil absorption of from about 150 to 300 millillters (ml~
per hundred grams of siliceous filler. The siliceous filler differs from
the conventional precipitated silica pigment described, for example, in
U.S. Patent 4,226,926 by the essentially hollow and spherical character
of the ultimate particles of the pigment. The aforesaid siliceous pig-
~ent can be prepared in accordance with the process described in U.S.
Patent 3,129,134.
~;~5~i9V
Detailed Description of the Invention
_
In accordance with the present invention, between about 10 and
about 90 weight percent, basis the polymerlc material, of esse~tially
hollow, spherical precipitated siliceous pigment is used to produce rein-
forced microporous polymeric battery separators. More particularly,
between about 20 and 75, e.g., between 30 and 60, weight percent of the
siliceous pigment is so used.
This siliceous pigment i8 composed of agglomerates of essen- - -
tially hollow spherical particles having a predominant hollow particle
size (diameter) of between 0.005 and 5.0 microns, e.g., between 0.01 and
1.0, or 0.01 and 0.20 microns. The siliceous filler is further character-
ized by a surface area of between about 50 and 250 square meters per
gram, (m2/gram) more typically between 75 and 200 m2/gram, and an oil
absorption of from about 150 to 300, e.g., from about 200 to 300, or from
about 230 to 270, ml of oil per hundred grams of slliceous filler. The
surfara area of the pigment can be det~rmined by the method of Brunauer,
Emmett, and Teller, J.Am. Chem. Soc., 60, 309 (1938), This method, which
is often referred to as the BET method, measures the absolute surface
area of a material by measuring the amount of gas adsorbed under special
conditions of low temperature and pressure. The BET surface areas
reported in the Examples were obtained using nitrogen as the gas adsorbed
and li~uid nitrogen temperatures (-196C.) and at a 0.2 relative pres-
sure. Oil absorption values are the volume o~ dibutylphthalate oil neces-
sary to wet 100 grams of the pigmentO These values can be obtained using
the method described in AS~ D2414-65.
The aforesaid siliceous pigment can be prepared in accordance
with the process described in U.S. Patent 3,129,134. In accordance with
the process therein described, the pigment is produced by precipitating
6~0
water insoluble siliceous product from an aqueous siliceous solution in
the presence of finely-divided particles of a water insoluble inorganic
salt, e.g., calcium carbonate - especially inorganlc salts of acids, the
anhydride of which is normally gaseous, for example, the inorganic salts
of carbonic acid. The water insoluble inorganic salt is then substan-
tially removed from the resulting insoluble siliceous precipitate by
treating the precipitate with acid, e.g, hydrochloric acid. This treat-
ment converts the cation of the insoluble inorganic salt into a water-
soluble salt of the treatmsnt acid and liberates the anion of the salt as
a gas. Thus, treatment of calcium carbonate solids in the silica product
slurry with hydrochloric acid, produces calcium chloride as the soluble
salt and carbon dioxide ~or carbonic acid).
More particularly, the aforesaid described siliceous pigments
are prepared by precipitating siliceous product in a slurry of f~nely-
divided water-insoluble carbonate salt, most notably calcium carbonate.
The particle size of the calcium carbonate or other similar water tnsolu-
ble salt of carbonic acid in the slurry should preferably approximate the
desired hollow spherical particle size of the precipitated siliceous pig-
ment to be used to prepare the battery separator. The calcium carbonate
can be preformed and slurried in the aqueous medium in which the precipi-
tation is accomplished. Alternatively, the calcium carbonate can be pre-
pared, in situ, by the reaction of calcium chloride with sodium carbonate
in the aqueous medium in which the precipitation is accomplished. Prod-
ucts produced utilizing insoluble carbonate salts formed in situ in the
reaction vessel are generally smaller in hollow spherical particle size
than those obtained using slurries of preformed water insoluble carbonate
salts.
-- 4 --
i90
From the physical appe~rance of the pigment, i.e., the substan-
tial absence of water insoluble siliceous material in the core of the
particles, the foregoing method apparen~ly precipitates ~ater insoluble
siliceous material upon the surface of the finely-divlded water-insoluble
carbonate salts, e.g., calcium carbonate particles. This carbonate parti-
cle is subsequently converted e.g., by acid treatment to water-soluble
components, thereby leaving an essentially hollow, spherical silica
particle.
The method of precipitating the water insoluble siliceous mate-
rial from solution described in U~S. 3,129,134 may include partially neu-
tralizing with hydrochloric acid (or like neutrallzing agent) the aqueous
solution of alkali metal, e.g., sodium, silicate. The extent of this
partial neutrali~ation is such tha~ the resulting aqueous solution will,
upon standing (sometimes for but a very brief duration), precipitate
water i~soluble siliceous material from the solutlon. Prior to develop-
ing such siliceous precipitate, ~recipitation of the pigmentary siliceous
material may be induced by introducing into the solution a precipita~e
inducing soluble inorganic metal salt, such as calcium chloride andjor
sodium chloride.
The essentially hollow, spherical siliceous pigment ~after
removal of th~ water insoluble inorganic salt) is a finely-divided floccu-
lated amorphous precipitated siliceous pigment. The pigment is in the
form of flocs or agglomerates of quite small particles of siliceous mate-
rial. The number average spherical particle size is below about p.5
micron in diameter and usually less than 0.3 micron in diameter but
rarely less than 0.01 micron in dlameter. A multiplicity of these small
particles are agglomerated together without complete loss of their indi-
vidual identities providing the pigment's flocculated state. Flocs can
~58~
range upwards of 40 microns in si~e, as measured at their longest
dlmension.
The degree to which these flocs persist (are not degradated
into smaller flocs) when the pigment is subjected to mechanical action,
i.e., milling, can vary. However, even those pigments which have their
average floc si~e altered by mechanical means still retain the flocculant
characteristic. This flocculated state appears predominantly in the form
of three~dimensional clusters, which may be likened to bunches of individ-
ual hollow grapes in which the particles in the floc are denoted by the
individual hollow grapes and the floc is represented by the cluster.
An important feature of these precipitated siliceous pigments
is the character of the ultimate particles. Such particles are composed
of an optically dense outer shell (shell-like structure~ of siliceous
material. The interior volume enclosed or within the shell is of much
lower optical density, e.g., below the optical denslty of water-insoluble
precipitated siliceous material under the high magnification of an elec-
tron microscope. The ultimate particles appear almost bubble-like and
spheroidal with the dif f erence in optical density between the inner and
outer volumes giving them the appearance of hollow particles.
Fluids, e.g., gases or liquids, may occupy the interior volumes
of the spherical particles, the dimensions of which are defined by the
inner surface of the particle's siliceous shell. When well dried, little
of any liqu~d, such as water, normally occupies or fills the interior
volume. Typically, the siliceous shell encloses or encases completely
the less optically dense interior. However, the shell is sufficiently
porous to allow remo~al of the water-insoluble carbonate salt. That is,
the shell is predominantlv continuous (but porous) - at least to the
extent that when the interior volume is a fluid, it is possible to remove
- 6 -
1~5~t;90
or replace the fluid. Larger particles may have a discontinuous shell
due to the non-uniform coating of large particles of ehe carbonate salt
or the coating of aggregates of carbonate salt particles, i.e., non-d s-
persed individual carbonate salt particles~ It is believed that the
shell's porosity is composed of indirect pathways from the outslde to the
inside of the shell circumventing the ultimate particles of siliceous
material that make up the shell.
Chemica~ly, the slliceous pigments have a substantial SiO2
content, usually at least 50 percent by weight SiO2 on an anhydrous
basis. Also commonly present are one or more metals, usually as their
metal oxides, including frequently an alkaline earth metal oxide such as
calcium oxide. The hollow spherical precipitated particles desirably
contain less than 2 weight percent of the alkaline earth metal (measured
as the oxide) for use in battery separators. Preferably, the alkaline
earth metal conten~ is less than 1 weight percent, more preferably less
than 0.5 weight percent and most preferably less than 0.1 weight per-
cent. The alkaline earth metal content of the sillceous pigment can be
reduced by treating the precipitated pigment with sufficient acid to con-
vert all of the alkali~e earth metal to soluble salt and by thoroughly
washing of the pigment ~after acid treatment and before drying).
After drying, the siliceous pigment is white, fluffy, pulveru-
lent and dry to the touch. Despite appearing dry, the pigment normally
contains water, e.g., between about 2 and 8 percent "free water" by
weight. Free water is that water which is removed from the pigment by
heating at 105C. for 24 hours. The pigment also contains "bound water",
which refers to that water removed by heating the pigment at ignltion
temperature, i.e., 1000C. to 1200C. for an extended period, e.g., 24
hours. Bound water can constitute hetween about 2 and 6 percent of the
pigment.
l;~SB~ 3~
The polymeric material into which the siliceous pigment is
incorporated to prepare the microporous battery separator can be any of
the conventional natural and synthetic polymeric materlals conventionally
used to fabricate battery separators. Among such materlals, there can be
mentioned natural rubber, styrene-butadiene rubber, nitrile-butadiene -
rubber, polyisoprene, high molecular weight olefins such as polyethylene,
polypropylene, polybutene, ethylene-propylene copolymers, ethylene-butene
copolymers, propylene-butene copolymers, ethylene-propylene-butene copoly-
mers, polyvinyl chloride and vinyl chloride-vinyl acetate copolymers.
Mixtures of such materials have also been used to prepare battery
separators.
Other conventional materials added to the polymeric material,
such as plasticizers, antioxidants, wettlng agents, carbon black and cur-
ing agents, e.g., sulfur, for rubbery polymeric materials may also be
added to the composition used to prepare the battery separator.
Battery separators incorporating the above-described hollow
spherical siliceous filler can be prepared in accordance with known tech-
niques for preparing such articles. A typical procedure for preparing a
battery separator utilizing a curable rubber is described in U.S. Patent
4,226,926. In that patent, the siliceous filler is rehydrated to levels
of between 65 and 75 percent by admixing the siliceous filler with
water. The resulting free flowing rehydrated silica powder is admixed
with the polymeric material, e.g., in a Banbury mixer. Thereafter, the
mixture (including any additional additives required for curing the poly-
meric particle) is milled on a 2-roll mill to produce a milled sheet.
The milled sheet is soaked in hot water and then calendered for con-
tours. Optionally a backing such as paper or a heat-bonded mat is added
to the milled sheet. The calendered slleet is then cut into appropriate
sizes.
1~c7c~6 J~ tt r/4 . ~ 8
Another similar procedure is described in U.S. Patent
3,351,4~5. There, the polymeric material, e.g., a polyolefin having a
molecular weight of at least 300,000, ls blended with the iner~ filler~
e.g., silica, and a plasticizer. The blend, which may also contain con-
ventlonal stabilizers or antioxidants, is molded or shaped, e.g., by
extrusion, calendaring, injection molding or compression, ir.to sheets.
Plasticizer and/or filler is removed from the sheet by soaking the sheet
in a suitable solvent, e.g., chlorinated hydrocarbons for a petroleum oil
plastici~er and water, ethanol, acetone, etc. for a polyethylene glycol
plasticizer.
The present invention is more particularly described in the
followin~ examples which are intended as illustrative only since numerous
modifications and variations thereln will be apparent to those skilled in
the art.
Example I
15 liters of an aqueous solution of sodium silicate
[Na20(SiO2)3 18] containing 20 grams per liter Na20 was fed at
the rate of 0.5 liters per minute to one arm of a tee tube. To the other
arm of the tee tube, was fed 15 liters of an aqueous solution of hydro-
chloric acid containing 11.8 grams per liter ~Cl at a rate of 0.5 liters
per minute. The resulting partially acidified sodium silicate solution
was charged to the upper portion of a suitable reaction vessel. Added
simul~aneously to the upper portion of the reaction vessel throu~h an
lnlet tube was 15 liters of a salt solution containing 0.48 moles per
liter of calcium chloride and 0.37 moles per liter of sodium chloride at
a rate of 0.5 liters per minute. Also added to the reaction vessel
through an inlet tube adjacent to the salt solution inlet tube was 15
~Z58~
liters of an aqueous solution containing 0.16 moles per liter of sodium
carbonate at a rate of 0.5 liters per minute. The reactant streams had a
temperature of about 23C. The reaction mixture collected for the first
4 minutes was discarded. The remaining reaction mixture slurry was trans-
ferred to a polyethylene lined vessel. This slurry was neutralized to a
pH of 2.0 with 1600 milliliters of 6 Normal hydrochloric acid. The acidi-
fied slurry was agitated with an air stirrer for 15 minutes and the pH of
the slurry readjusted to 7.5 over 15 minutes with 138n milliliters of 2.5
Normal sodium hydroxide. The slurry was heat aged at 105~C. in an oven
overnight.
Thereafter, the slurry was removed from the oven and filtered.
The filter cake was washed with 72 liters of distilled water to wash the
cake free of chloride ion. The filter cake was broken-up, placed in
stainless steel trays and dried overnight in an oven at 105C. The drled
pigment was removed from the oven, rehumidified and micropulverized
through a 0.020 inch round screen.
Optical microscopic examination of the micropulveri~ed ~aterial
revealed that relatively large calcium carbonate particles were still
present in the product. The amount of calcium present in the product
(measured as ca~cium oxide) was found by chemical analysis to be 4.49
percent.
The product was reslurried in 10-12 liters of distilled water
and sufficient 6 Normal hydrochloric acid added to the slurry to lower
the pH to 2Ø The slurry was s$irred for four hours while ~aintaining
the pH at 2Ø A total of 475 milliliters of hydrochloric acid was added
to the slurry. Thereafter, the slurry was neutralized with 630 milli-
liters of 2.5 Normal sodium h~droxide to raise the pH of the slurry to
7.65. The slurry was filtered and the filter cake washed with 24 liters
-- 10 --
.~S8~;9~
of distilled water. The washed filter cake was dried overnight at 105C.
in an oven and thereafter micropulveriz.ed through a 0.020 lnch round
screen. The micropulveri~ed product was rehumidified by exposure to
ambient alr over a weekend.
The resulting product was submitted for surface area and oil
absorption determinations, and elemental X-ray analysis. Results are
tabulated in Table I.
Example II
18 liters of an aqueous solution of sodium silicate
[~a2O(SiO2)3 18] containing 10.5 grams per liter Na20 was fed at
the rate of 0.5 liters per minute to one arm of a tee tube. To the other
arm of the tee tube was fed 18 liters of hydrochloric acid containing
0.187 grams per liter HCl at a rate of 0.5 liters per mlnu~e. Simultane-
ously, 36 liters of a salt solution (calcium chloride plus sodium chlo-
ride) containing 57 grams/liter o~ Camel-Wite Super~ ~round calcium car-
bonate of approximately 3 micron particles were added to the reaction
vessel. The salt solution contained 0.169 moles per liter of calcium
chloride and 0.128 moles per liter of sodium chloride. The salt slurry
was introduced into the reaction zone at a rate of 1.0 liters per
minute. The temperature in the reaction vessel was about 18C. The
first 4 1/2 minutes of slurry produced was discarded and thereafter the
resulting slurry collected. The pH of the product slurry after addition
of all of the reactants was 9Ø The pH of the slurry was adjusted to
2.0 with 6.0 liters of 6 ~ormal hydrochloric acid. The acidified slurry
was stirrPd for 20 minutes and thereafter the pH adjusted to 8.0 with
2520 milliliters of 2.5 Normal sodium hydroxide. The resulting slurry
was heat aged ovPrnight at 105C.
~S~
The aged slurry was removed~ ~acuum filtered and the filter
cake washed ~ith 112 liters of dlstilled water. The filter cake was
broken-up and dried overnight in an oven at 105C. The dried product was
rehumid~fied and then micropulverized through a 0.020 inch round
screen.
Optical microscopic examination of the milled pigment showed
large calcium carbonate particles still present in the sample. The
amount of calcium present (measured as calcium oxide) was found to be
13.3~ weight percent CaO. Accordingly, the product was reslurried in 10
to 12 liters of distilled water and 960 milliliters of 6 Normal hydro-
chloric acid added to the slurry to reduce the pH to 2Ø The acidified
slurry was stirred for 4 hours and thereafter 310 milliliters of 2.5 Nor-
mal sodium hydroxide added to raise the pll of the slurry to 7.70. The
neutralized slurry was filtered and the filter cake washed with 32 liters
of distilled water. The filter cake was broken-up and dried overnight in
an oven at 105C. The dried pigment was micropulverized through a 0.020
inch round screen. The resulting product was submitted for surface area
and oil absorption determinations, and elemental Y.-ray analysis. Results
are tabulated in Table I.
Example III
In a manner similar to Example I, 36 liters of an aqueous solu-
tion of sodium silicate [Na2O(SiO2)3 06] contailling 20 grams per
liter NazO was charged at a rate of 0.5 liters per minute into one arm
of a tee tube. Through the other arm of the tee tube was charged 36
liters of hydrochloric acid contalning 11.8 grams per liter at a rate of
0.5 liters per minute. The concentration of the calcium chloride/sodium
chloride salt solution and sodium carbonate solution were the same as in
- 12 -
~s~
Example I e~cept that 36 liters of each solution were lneroduced in~o the
reaction vessel at a rate of O.S l$ters per minute. The temperature
within the reaction vessel was about 23-24C. The resulting product
slurry had a pH oE 9.15. The pH of the slurry was adjusted to 2.0 with
3.9 liters of 6 Normal hydrochloric acid and stirred for 3 hours at that
pH. Subsequently, the acidified slurry was neutralized to a pH of 7.8
with 2.4 liters oE 2.5 Normal sodium hydroxide. The neutralized slurry
was aged overnight in an oven at 105C. and the aged slurry filtered and
washed with 96 liters of dist~lled water. The washed filter cake was
dried overnight in a 105C. ovan. The dried product was rehumidified at
room temperature and micropulverized (hammer milled) through a 0.01 inch
screen~
The resulting product was submitted for X-Ray elemental analy-
sis and surface area and oil absorption determinations. Results are tabu-
lated ln Table I.
Examp]e I~ -
The procedure of Example II was followed utilizing 18 liters of
the aqueous sodium silica~e solution described in Example III. The
sodium silicate solution was partially neutralized with 1~ liters of
hydrochloric acid containin~ 12.9 ~rams per liter of ~Cl charged at a
rate of 0.5 llters per minute. 36 liters of a salt solution containing
0.65 moles per liter calcium chloride and 0.51 moles per liter sodium
chloride was introduced into the reaction vessel at a rate of 1 Liter per
mlnute. The salt solution contained 0.57 moles (57 grams) p~r liter of
preformed calcium carbonate having an average particle size of 0.75
microns. The temperature in the reaction vessel was about 19C. The p~1
of the resulting slurry following addition of all of the reactants was
3! ;~5~ ;<3(~
8.6. The pH of the slurry was ad~usted to 2.0 wlth 4,82 liters of 6 Nor-
mal hydrochloric acid and the acidified slurry stirred for 4 hours at a
pH of 2. Subsequently, the aged acidified slurry was neutrali~ed wlth
1.34 liters of 2.5 Normal sodium hydroxide to a pH of 7.8. The neutral-
ized slurry was aged overnight at 105C. and subse~uently filtered. The
filter cake was washed with 128 liters of distilled water and the washed
fllter cake dried overnight in a 105C. oven. The dried product was
rehumidified at room temperature and micropulverized through a 0.01 inch
screen. The milled product was submitted for physical and chemical analy-
sis. Results are tabulated in Table I.
TABLE I
Example Particle Size* Surfa~e Oil Abs, ~ X-Ray Analysis, Wt. %
No. Range, um Area~ m ¦g ml/100 g H20 Ca Cl Fe Na Al M
I 0.009-0.07133 210 3.46 0.50 0.05 0.12 0.19 0.34 <0.0
II 0.1-5.0 80 189 4.58 1.4 0.41 0.40 0.13 0.43 0.2
III 0.02~0.08102 249 4.72 1.5 0.12 0.09 0.33 0.29 0.2
IV 0.2-1.0 57 263 5.65 1.6 0.55 0.15 0.54 0.28 0.6
* Predominan~ Ultimate Particles
~Jater content was determined by measuring weight loss of the sample
after heating at 105C.
The product of ~xample III was substituted for a conventional
precipitated silica pigment in a battery separator and the resulting sepa-
rator was reported to exhibit a reduction in electrical resistance of
~rom 15 to 20 percent.
From the foregoing examples, it is clear that various proce-
dures and reagents may be used in preparing the hollow silica plgments
descrlbed herein. Any suitable water soluble alkali metal sillcate, for
examples, serves as a source of the S10~ content of the ultimate pig-
5~:;9~
ment product. Sodium silicate containing from ~ to 4 moles SlO2 permole of Na20 is the more widely available and used material. Others,
including potassium silicate, lithium silicate and sodium potassium sili-
cate containing from 1 to 5 moles of SiO2 per mole of alkali metal
oxide, however, may also be used.
The salt which induces precipitntion of the water insoluble
siliceous material also can be varied. It is usually preferably that the
salt be a water soluble halide, notably a chloride, such as calcium chlo
ride. Among the salts which may be so used include: sodium chloride,
barium chloride, strontium chloride, zinc chloride, calcium bromide,
sodium ioaide, water soluble metal salts of strong acids, e.g., acids
having an ionizatlon constan~ of at least 1 X 10 2, such as metal
nitrates exemplified by calcium nitrate and sodium nitrate, and metal
sulfates such as sodium sulfate.
As with the other reagents, there is latitude in the acidic
material used to partlally neutralize the aqueous solution of alkali
metal silicate. Acids such as hydrochloric acid, sulfurlc acid, nitric
acid, acetic acid, sulfurous acid, phosphorlc acid and carbonic acid (or
thelr anhydrides) can be mentioned. In the main, preference is for
acidic materlals, the anions of which do not form water insoluble mate-
rials under the prevailing conditions with alkali metalsn
Acids or acidic materials that may be used in treating the
slurry of precipitated water insoluble siliceous material for the purpose
of water solubilizing the water insoluble non-siliceous materia~, such
as calcium carbonate may also vary. Most typically 9 such acids are those
which, upon reaction with the aforesaid water insoluble material will
provide a water soluble salt of the cation aDd result in generating 2n
anhydride of the anion of the water insoluble material, such as carbon
- 15 -
5~6"3~
dio~ide t~hen the material is calcium carbonate. For this purpose, strong
mineral acids such as hydrochloric~ and nitric acids are suggested.
While the invention has been described in deta$1 with respect
to certain embodiments thereof, it is to be understood that the invention
is not intended to be limited to such details except as and insofar as
they appear in the appended claims.
