PROCESS FOR MAKI NG Tl LES
TEC HNIC AL FIELD
The present disclosure relates to a process for making ceram ic tiles
characterized by the addition to the ceram ic raw materials of an aqueous
slurry comprising a swellable clay of the smectite fam ily, a binder and a
water-soluble salt of a monovalent cation.
PEIO R ART
The production of ceramic tiles generally involves the following steps:
I) mixing of the ceram ic raw materials;
I I) wet grinding of the ceram ic raw materials;
I I I) spray drying of the thus obtained ceram ic slip;
IV) form green tiles by pressing the powdery intermediate;
V) drying the green tiles;
VI) glazing the upper surface of the dried green tiles;
VI I) firing the glazed green tiles.
The ceram ic raw materials useful for the preparation of ceram ic slips are
the typical ceram ic raw materials and are of two basic types:
• clayey materials (typically china clays and red vitrifiable clays) ;
• complementary materials (typically feldspars, feldspathoids,
feldspathic sands, quartzes, pegm atites, etc.) , having melting and/or
inert features.
The purpose of grinding is to effect size reduction of the ceramic raw
materials and to homogenise them until a final constant particle-size
distribution has been achieved; generally speaking the residue after
gri nding is aro und 0.5- 10 % by weig ht (% wt) on a 63 micron s (230
mesh ) sieve , depend ing on t he nat ure of t he ceram ic mat eri als .
Wet gr inding prov id es wet gr inded ceram ic raw materials, also cal led
ceram ic slips, contai ning abou t 30-40 % wt of water .
The subseq uent st ep is sp ray dry ing of t he ceram ic sl ip . The purpose of
spray dry ing is to ach ieve a part ial evapo rat ion of t he wat er contai ned in
t he sl ip (redu ct ion of wat er conte nt to 4-7 % wt ) t oget her wit h t he
form at ion of sph ero id part icl es.
Typical part icles size dist r ibut ion of spray- dried powders for vit rifi ed
single- f ired t iles is 70- 80 % wt of part icles in t he range from 425 to 180
micron s .
The purpose of form ing t he t ile body by pressi ng is to obtain t he utmost
possi ble den sificat ion of t he powders on green t iles, com pat ibly wit h
problem s of black core or degassi ng , which may t urn up during f iring;
gen eral ly speaki ng, t he specific form ing pressu re for t he bod ies is arou nd
200-450 Kg/cm 2.
Dry ing is t he processi ng phase which eliminates t he resid ual pressi ng
moist ure in t he newly form ed t iles; t he t ile bod ies coming out of t he
presses are collected by rol ler lines and sen t to t he dryers, prov id ed wit h
inside channels dispen sing hot ai r to t he dry ing zon e.
Glazi ng may be perfo rmed using t he usual dry or wet appl icat ion
tech niques.
Fi ring is perfo rmed in a kil n using pre- defin ed f ir ing cycles; t he f iring
cycles and temperat ures gen erally fall respect ive ly wit hin t he rang e of
20-60' and 1100- 1250 °C, depen ding of t he nat ure of t he ceram ic
masses to be f ired and on t he size of t he t iles t hemselves.
Fo rm ing and dry ing of t he ceram ic green t ile bodi es represen ts crit ical
operat ions in t he manufact ure of t he art icles. Add it ives are com monly
added in t he preced ing st eps in order to red uce defect s gen erated during
pressi ng and dryi ng . Typical add it ives are binders and plast icizers.
Bind ers are added for t he specific purpose of cem ent ing toget her t he
powd ery raw mat erials and in creasin g t he mech anical resist ance of t he
dried green t iles. They are oft en organ ic in nat ure (such as molasses,
ligninsu lfonates, starch and derivat ives t hereof) ; inorgan ic binders are
also known and used (binder clays) .
Plast icizers are added for t he specific purpose of increasi ng t he capacity
of t he slips to change perm anent ly in size and shape dur ing t he for ming
of t he t iles. Known organ ic plast icizer are glyco ls, such as polyet hylen e
glyco ls, polyv inyl alco hols and polyacrylates.
Unfort unately t he addit ion of large amount of organ ic add it ives increases
t he organ ic matt er content in ceram ic bod ies and experi ence has shown
t hat pressed t ile bodi es employ ing t oo much organ ic are quite subj ect to
black cori ng problem s.
Also in org anic plast icize rs are known . Exam ples of inorganic plast icizers
are specific clays, such as t he bal l clays or clays bel ong ing to t he grou p
of il lit e-chlorite and/ or illit e- kaol init e clays, but t heir use is limit ed by
t heir relat ive high cost and per iod ical short ages.
It is known that swelling clays of the smectite family, such as the
bentonites, exhibit plasticity and binding properties and that their
addition to the ceram ic raw materials, in the manufacture of tiles, will
increase both the green and the dry strength. For these reasons
bentonites are occasionally combined in small amounts as a powder with
the ceram ic raw material before grinding. However, there is wide
variation in the chemistries of bentonites and, before their addition to the
slips, they must be tested in laboratory and carefully controlled on the
plant because their strong effect on the viscosity and the rheology
characteristics of the ceram ic mixtures can produce dramatic results on
the grinding process. Moreover bentonites shrink more during drying and
thus potentially crack more, so only a lim ited amount can be added.
It has been now found that a swelling clay of the smectite fam ily, a
water-soluble salt of a monovalent cation and specific organic binders
may be formulated in liquid form , as a particular pourable aqueous
slurry, which can be added without difficulty to the ceram ic raw
materials. The aqueous slurry of the invention acts as a plasticizer and as
a binder, does not cause the problems of black coring, has a predictable
behavior and, since it has a little effect on the viscosity of ceram ic raw
material mixtures or of the ceramic slips, does not require any
prelim inary laboratory test and increases the strength of both the green
and the dried tile bodies. Moreover the slurry, being a easily pourable
liquid, may be added not only during the mixing of the raw materials but
also during the let down phase after the grinding step without causing
form at ion of gel s or resid ues which wou ld req uire a furt her long and
difficu lt f ilt rat ion st ep .
Slu rries of swel ling clays are known and are used , for exam ple , in t he
papermaki ng secto r .
US 5,0153,34 describes a composit ion com prising a wate r dispersible
colloidal siliceo us mat erial , such as a swel ling clay , in int imate associat ion
wit h a low molecu lar weig ht wate r soluble high charge den sity organic
polymer , such as a polyacry lic acid or a polyam ine. The polymer
concen t rat ion is fro m 0.5 to 25% on t he dry weig ht of t he siliceou s
mater ial .
US 5,266,538 prov id es an elevated sol id s aq ueou s slurry of a smect it e
clay , e.g. up to 50 % by weug ht of sol ids, con taining an effect ive
concen t rat ion of a monovalen t salt . The monoval ent salt is preferably
sod ium chlor id e.
US2 007131372 prov id es aqu eou s slurries of phyllosi licate- con taining
material and a sizin g agent , havi ng a fin al Hercu les apparent viscosity
below 100 cps. These slu rries may have a cont ent of from 20 to 55 % wt
of solid phy llosi licat e mater ial . Exam ples of sizin g agen ts include vario us
form s of rosi n, alo ne or in combinat ion wit h ot her mat eri als, such as
ligninsu lfonates, tal l- oil fatty acids, sty ren e com pou nds, f luorocarbo ns,
acry lic emulsions and sty ren e acry lat es, stearates and st earic acid .
EP1 329484 descri bes a clay slurry comprisi ng a clay of t he smect ite
fam ily , a defi ned phosph onate add it ive and water , which is part icu lar ly
usefu l as an ant i-bleed add it ive for con cret e. The smect ite clay slurry
contains at least 2 % by weight of clay and from about 0.5 to 15 % wt
based on the weight of the smectite clay of one or more phosphonates.
As far as the Applicant knows, none has described the use of the slurries
of the present disclosure in the production of ceram ic tiles nor their
specific formulation.
DESC Ί N 0 FTHE INVE TON
It is a fundam ental object of the present invention an aqueous slurry
com prising:
a) from 5 to 30 % by weight of a swelling clay of the smectite family;
b) from 10 to 30 % by weight of a binder chosen among lignin
sulfonates, naphthalene sulfonate-formaldehyde condensate salts,
mono- and oligo-saccharides, water-soluble starches, water-soluble
cellulose derivatives, and mixture thereof;
c) from 0.1 to 10 % by weight of a water-soluble salt of a monovalent
cation.
It is another object of the invention a process for making ceram ic tiles
com prising the following steps:
I) mixing of the ceram ic raw materials;
I I) wet grinding of the ceram ic raw materials;
I I I) spray drying of the thus obtained ceram ic slip
characterized by the addition during step I) or after step I I) and before
step I I I) of from 0.2 to 3 % by weight, preferably from 0.4 t o 2 % by
weight, of said aqueous slurry.
DETAILED DESC Ί N 0 F THE INVENTIO N
Preferably the aqueous slurry comprises:
a) from 10 to 20 % by weight of a swelling clay of the smectite fam ily;
b) from 15 to 25 % by weight of said binder;
c) from 0.5 to 5 % by weight of said salt of a monovalent cation.
Usually, the aqueous slurry of the invention comprises from 35 to 84.5 %
by weight of water.
The swelling clays of the smectite family belong to a well known fam ily of
three-layer clay minerals containing a central layer of alum ina or
magnesia octahedra sandwiched between two layers of silica tetrahedra
and have an idealized form ula based on that of pyrophillite which has
been modified by the replacement of some of the 3, Si+4, or Mg+2 by
cations of lower valency to give an overall anionic lattice charge. The
swelling clays of the smectite fam ily include montmorillonite, which
includes bentonite, beidellite, nontronite, saponite and hectorite. The
swelling clays usually have a cation exchange capacity of from 80 to 150
meq/ 100 g dry mineral and can be dispersed in water relatively easily.
For use according to the present invention, the swelling clay of the
smectite fam ily is preferably in the sodium or lithium form , which may
occur naturally, but is more frequently obtained by cation exchange of
naturally occurring alkaline earth clays, or in the hydrogen form which is
obtainable by mineral acid treatment of alkali metal or alkaline earth
metal clays. Such sodium , lithium or hydrogen-form clays generally have
t he property of increasi ng t heir basal spaci ng when hydrated to favo r t he
phenomenon known as swell ing.
Fo r t he real izat ion of t he prese nt inven t ion, ben tonit e is t he preferred
swelling clay of t he smect ite fam ily , sod ium bentonite is part icu larly
prefer red .
The binders b) suit able for t he real izat ion of t he presen t invent ion are
ligninsulfonate , naph t halen e sulfo nate- form ald ehyde cond ensate salt s,
mono- and oligo- sacch arides, water- sol uble st arch es, wat er- soluble
cellulose der ivat iv es, such as carboxy met hyl cel lulose and hydroxyet hyl
cellulose , and mixt ure t hereof . Ex amples of mono- and olig o- sacch ar id es
are sugars, such as glucose and sucrose ; sugar alcoh ols, such as sorb ito l;
dext rins and maltodext r ins . These binders are commonly used in t he fie ld
and wel l known to t he expert in t he art .
Part icu lar ly preferred binders for t he real izat ion of t he inven t ion are
sod ium or pot assi um ligninsu lfonates.
Lign in sulfon ates are a by- prod uct of t he prod uct ion of wood pulp. As t he
organ ic lig nin molecu le com bines wit h st rongly pol ar sulfon ic acid grou ps
dur ing sulfit e pulping , lignin sulfon at es are read ily solu ble in water in t he
form of t hei r sod ium, calci um or ammonia salt s. Lig ninsulfo nates are
avai lable as yel lowish powd ers having variabl e com posit ions and also
var iable molecu lar dimensions . A typ ical weig ht ave rage molecu lar weig ht
of t he lig ninsu lfonates is abou t 30,000 dalto n ( Da) and its typ ical
number ave rage molecu lar weig ht is abo ut 3,000 dalt on .
Naph t halen e sulfonate-form ald ehyde condensat e salts, also cal led NSF,
have been known for som e t ime and have been fully descri bed also as
dispersi ng agen ts in diffe ren t sect ors. In gen eral t hese mater ials are
made by conden sin g molt en naph t halene wit h fuming sulfu ric acid t o
form napht halen e sulfon ic acid derivat ives havi ng vary ing posit ion
isom ers. The sulfonic acid derivat ive is t hen con den sed wit h wat er and
form aldeh yde at temperat ures of abou t 90 °C and t hereaft er convert ed
to a salt by t he add it ion of alkal i metal or ammonium hydrox ides or
carbon at es. The weig ht- average molecu lar weigh t of t he napht halen e
su lfon at e formalde hyde con densate salts, suitable for t he real izat ion of
t he presen t invent ion , is preferably arou nd 10,000 Da.
The carboxym et hyl cel lulose suit able for t he real izat ion of t he presen t
invent ion can be chosen among t hose com monly used in t he ceram ic field
and known to t hose ex pert in t he art . The carboxym et hyl cel lulose
prefer red for t he real izat ion of t he present inven t ion has degree of
subst it ut ion com pr ised between 0.5 and 1.5, more preferab ly between
0.6 and 1.2. Preferab ly its Broo kfi eld® LVT viscosity , at 2% wt in wate r ,
60 rpm and 20 °C, is from 5 to 300 mPa- s, more preferab ly fro m 5 to 50
mPa s.
Prefer red binders are lignin sulfon at es, naph t halen e su lfon ateform
aldeh yde conde nsat e salt s , sugars, sugar alcoh ols, carboxy met hy l
cellulose , and mix t ure t hereof.
The aqueou s slurry com prises also a water soluble salt of a monovalen t
cat ion c) , which red uces t he swel ling capacity of t he smect ite clay and
lowers the viscosity of the slurry and subsequently of the ceram ic slips.
Salt containing divalent or higher valency cations (for instance calcium)
can be used in som e instances but these divalent ions tend to exchange
with the monovalent ions that are present in the swellable clay initially
and this can inhibit the subsequent swelling of the clay. It is generally
preferred therefore that the cation of the salt is monovalent, for example
ammonium or alkali metal. Exam ples of useful salts are ammonium salts
(in particular NH4
+ , tetra-Ci-C -alkyl amm onium and tetra-Ci-C 4-alkenyl
amm onium salts, in which one or more of the alkyl or alkenyl groups is
substituted by an -OH group) or alkali metal salts of chloride, brom ide,
phosphate (monobasic, dibasic and tribasic phosphate) and mixtures
thereof. Specific examples are sodium or potassium chloride, sodium or
potassium brom ide, monobasic sodium or potassium phosphate, dibasic
sodium or potassium phosphate, tetraalkyl am monium chloride, choline
chloride and mixtures thereof.
Preferably the monovalent cation is sodium , potassium or choline.
The salt is preferably sodium chloride, potassium chloride and choline
chloride, more preferably potassium chloride.
In a preferred embodim ent the aqueous slurry of the invention also
com prises from 0.5 to 5 % by weight, preferably from 1 to 3% by weight,
of a dispersant, chosen among those com monly used in the field.
Examples of dispersant are (meth)acrylic acid polymers, usually provided
as sodium salt; phosphates and polyphosphates, such as sodium
tripolyphosphate; sodium metasilicate; sodium di-silicate; liquid sodium
si licate ; and mixt ures thereof . Part icu lar ly preferred dispe rsan ts are
(met h)acry lic acid polymers wit h a weight averag e molecu lar weig ht
below 20,000 Da, and preferably bel ow 10,000 Da, for instance from
1,000 to 6,000 Da.
Common ceram ic add it ives can be also presen t in the aq ueous slurry of
t he inven t ion. Ex ample of add it ives are ant ifoam s, perfu mes,
preservat ives, dyes and the like.
The aqueou s slurry is prepared by f irst disso lv ing in water the salt c) ,
t he binder b) and the opt ional add it ives and thereu pon dispersi ng in t he
solut ion t he swellable clay of the smect ite fam ily a) . The clay- binder
mixt ure is st irred wit h minim um shear , preferab ly as the clay is added . It
has been found that the lower the shear of mix ing, the higher the solids
content that can be reach ed . Any mix ing dev ice capabl e of produ cing
low- shear mix ing can be employed .
Usu ally , the f inal aq ueou s slurry has a Broo kfi eld viscosity (25 °C, 20
rpm ) of below 3,000 mPa- s, preferab ly from 500 to 1,500 mPa- s.
It is import ant to not e that the slurri es of the presen t invent ion have low
viscosity and high solids content . They are also st able and ch aract erized
by prol onged shelf lives.
The aqueou s slu rries described above can be used for maki ng ceram ic
t iles accordi ng to the process of presen t invent ion. As already ment ion ed
the slu rries can be added in st ep I) .
Accord ing to this embodiment , the combinat ion of the ceram ic raw
materials and t he aq ueo us slurry is typical ly accomplish ed by mix ing
carefully the ceram ic raw materials and the other additives such as
deflocculants with the slurry to form a hom ogeneous mixture.
The mixture of the ceram ic raw materials is then subjected to wet
grinding. This step may be performed using either the continuous or the
discontinuous process.
At the end of the grinding, the slips are sieved and sent to storage vats,
from where they are pumped to an atom izer. The aqueous slurries of the
invention can be also added to the slips any moment between grinding
and spray-drying, even directly in the transfer line between the storage
vats and the atom izer.
In step I I I) , the slips are dried as they are heated in the atom izer by a
rising hot air colum n , form ing small, free flowing granules that result in a
powder suitable for form ing.
The process for the production of ceram ic tiles further comprises the
following steps: pressing the powdery interm ediate to form green tiles,
drying the green tiles, glazing the upper surface of the dried green tiles
and finally firing the glazed tile bodies. These subsequent steps for the
preparation of ceram ic tiles can be accom plished by conventional
techniques and procedures.
The tile making process of the invention has several advantages
com pared to prior art processes of making ceram ic tiles using directly a
swellable clay of the smectite fam ily. In particular, the performance is
superior to that which is obtainable using the corresponding smectite in
powder and without the inconvenience of having to handle powder.
The process of the invention is suitable for the production of any kind of
ceram ic tile, such as wall tiles, floor tiles, stoneware, porcelain
stoneware, rustic stoneware, earthenware tile, mosaic tiles, which can be
both single and double fired.
The following non-lim iting examples illustrate exem plary aqueous slurries
and process using the slurries in accordance with the present invention.
EXAMPLES
Examples 1-3
Three aqueous slurries according to the invention were prepared with the
com mercially available components reported in Table 1.
Table 1
The slurries were prepared according to the following procedure:
• dissolve the salt in water;
• add the binder;
• dissolve under stirring, then add the dispersant and the other
additives (if any) ;
• after 5 minutes, gradually pour under stirring the bentonite into the
mixture;
• after 10 minutes of homogenization, sieve the slurry on a 100 micron
sieve.
Dissolution test
The effect of the slurries was evaluated on a ceram ic raw material
mixtures before grinding. The mixtures were prepared with the
com mercially available ceram ic raw materials reported in Table 2 for Tile
4.
The effect of the suspensions were evaluated by determ ining the Ford
viscosity (ASTM Standard Method D 1200-1 0) on a mixture prepared
without any additive (blank), on a mixture prepared with 1% by weight
of the slurry of Example 3 (Example 3A) and on a mixture prepared with
0.2% by weight of the same bentonite powder of Example 3 (Exam ple B,
with the sam e amount of bentonite of the slurry of Example 3A). The
mixtures were hom ogenized by means of high speed mechanical stirrer
equipped with a eight blades impeller, working at 320 rpm for 10
minutes.
The following results were obtained:
* Comparative
The resu lt s show t he excel lent st abil ity of t he viscosity of t he mixt ure of
ceram ic raw mater ials compri sing t he slurry of Exam ple 3.
The use of t he slurries acco rding to t he inven t ion allows to avoid hig h
viscosit ies and t he problem s t hat t hey wou ld create , such as difficu lt ies in
gri nd ing and in movi ng t he slips t hrough t he var ious steps of t he process.
An analog ous t est was perfo rmed on ceram ic slips obt ained by grinding
t he ceram ic raw mat eri als descr ibed in Table 2 for Ti le 1.
The t est was perfo rmed on a sli p wit hout add it ive (blank) , a sl ip wit h 1%
by weig ht of slurry of Ex ample 3 (Exam ple 3B) and a slip wit h 0.2 % by
weigh t of t he sam e ben tonit e powder of t he Exam ple 3 (Exam ple C, wit h
t he sam e amount of bento nit e of t he slu rry of Ex ample 3B) . The mix t ures
were homogen ized by mean s of high speed mech anical st irrer equipped
wit h a eig ht blades impel ler , worki ng at 320 rpm for 10 minutes.
After homog enizat ion, 250 g of each sli p were screen ed on a tared 63
micron s ASTM sieve ( 100 mesh ) and t he amount of und issolved mater ial
(resid ue) was dete rm ined by weigh t differen ce aft er dryi ng in oven at
105 °C for 2 hours.
The fol low ing resu lts were obt ained :
* Com parat ive
The resu lt s show t he mixt ure of ceram ic raw mat erials con taining t he
slurry of Ex ample 3 has a lower conten t of residu e.
The presen ce of high con cen t rat ions of resid ues creates probl ems in t he
subseq uent steps of t he process and forces t he user to f ilt rate t he slip a
seco nd t ime before t he spray- drying .
Strength test
The perform ances of t he slurri es of t he invent ion were det erm ined on
t iles bod ies prepared wit h t he commercially avai lable ceram ic raw
materials repo rt ed in Table 2, where in (part s) mean s (part s by weig ht )
and (%wt) mean s t he percen tage by weig ht of t he
Table 2
The slurries of Exam ples 1-3 were added to t he ceram ic slips obt ained by
gri nd ing t he ceram ic raw mat erials and caref ully homogenized usi ng a
mech anical st irrer .
After homogenization, the slips were conditioned at 75-80 °C in oven for
one night and grinded again to get particles with size below 0.75 mm.
At the end of the grinding process, the water content of the ceram ic slips
was brought to about 6 % by weight.
Green tile bodies (5 cm x 10 cm , 0.5 cm thick) were prepared by means
of a laboratory hydraulic press (Nannetti, Mod. Mignon SS/EA) applying a
pressure of about 300 Kg/cm 2 for wall tile bodies (Tile 1 -3) and about
400 Kg/cm 2 for standard gres tile bodies (Tile 4-7) .
Comparative green tiles were prepared with the same procedure and with
the sole ceram ic raw materials.
The modulus of rupture (MOR) of the green tile bodies was determ ined
according to the International Standard Test Method ISO 10545-4, using
a laboratory flexim eter (Nannetti, Mod. FM96) .
The MOR of the dry tile bodies was determined on the remaining tile
bodies after drying in oven for one night at 110°C.
The modulus of rupture is an index of the strength of the tile bodies. The
results expressed as % increase of the strength of the tile bodies
prepared according to the invention compared to the strength of the
com parative tile bodies are reported in Table 3.
Table 3
Tile 1 Tile 2 Tile 3 Tile 4 Tile 5 Tile 6 Tile 7
%Green Strength +14,1 +35,3 +70,6 +11,1 +13,1 +5,0 +6,0
%Dry Strength +41,8 +49,1 +75,3 +19,4 +43,5 +11,4 +16,8
CLAMS
1) Process for making ceram ic tiles com prising the following steps:
I) mixing the ceram ic raw materials;
I I) wet grinding of the ceram ic raw materials;
I I I) spray drying of the thus obtained slip
characterized by the addition in step I) or after step I I) and before
step I I I) of from 0.2 to 3% by weight of an aqueous slurry
com prising:
a) from 5 to 30% by weight of a swelling clay of the smectite family;
b) from 10 to 30% by weight of a binder chosen among lignin
sulfonates, naphthalene sulfonate-formaldehyde condensate salts,
mono- and oligo-saccharides, water-soluble starches, watersoluble
cellulose derivatives and mixture thereof;
c) from 0.1 to 10% by weight of a water-soluble salt of a
monovalent cation.
2) The process of claim 1, characterized by the addition of from 0.4 to
2% by weight of said aqueous slurry.
3) Aqueous slurry comprising:
a) from 5 to 30% by weight of a swelling clay of the smectite family;
b) from 10 to 30% by weight of a binder chosen among lignin
sulfonates, naphthalene sulfonate-formaldehyde condensate salts,
mono- and oligo-saccharides, water-soluble cellulose derivatives
and mixture thereof;
c) from 0.1 to 10% by weight of a water-soluble salt of a
monovalent cation.
4) The aqueous slurry of claim 3, comprising:
a) from 10 to 20% by weight of a swelling clay of the smectite
fam ily;
b) from 15 to 25% by weight of said binder;
c) from 0.5 to 5% by weight of said salt of a monovalent cation.
5) The aqueous slurry of claim 3 , wherein said aqueous slurry further
com prises from 0.5 to 5% by weight of a dispersant chosen among
(meth)acrylic acid polym er, phosphates and polyphosphates, sodium
metasilicate, sodium di-silicate, liquid sodium silicate and mixtures
thereof.
6) The aqueous slurry of claim 5, wherein said dispersant is a
(meth)acrylic acid polymer with a weight average molecular weight
below 20,000 Da.
7) The aqueous slurry of claim 3, wherein said salt of a monovalent
cation c) is chosen among chloride, bromide or phosphate of
amm onium or alkali metal salts, or mixtures thereof.
8) The aqueous slurry according to claims from 3 to 7, wherein said
swelling clay of the smectite fam ily a) is a bentonite.