Process for bleaching and increasing the ion-exchange capability of sepiolite

The present invention refers to a process for bleaching and increasing the ion-exchange capability of the sepiolite mineral, basically consisting of treating, at high pH and temperature, the sepiolite with a basic salt solution, which results in bleaching the starting material by dissolution of the organic material present in it, and in increasing the ion-exchange capability by a structural transformation of sepiolite. The bleaching can be optionally completed with a reduction treatment with sodium dithionite or with other reducing agents capable of reducing Fe.sup.3+ to Fe.sup.2+.

TECHNICAL FIELD OF THE INVENTION 
The present invention falls into the field of treatment of sepiolite with a 
basic salt, at high pH and temperature, so as to become bleached and to 
increase its ion-exchange capability. 
PRIOR ART 
Sepiolite is a mineral belonging to the clay family known as 
sepiolite-palygorskite. Sepiolite structure has been described by Nagy and 
Bradley (1955). Sepiolite is a hydrous magnesium silicate, with a fibrous 
structure, for which different structural formulas have been proposed, 
such as Si.sub.12 O.sub.30 Mg.sub.8 H.sub.x (OH).sub.y (H.sub.2 O).sub.z, 
with stoichiometric variations in the number of protons, surface hydroxyl 
groups and crystallization water molecules, depending on the origin and 
taking into account the peculiar properties thereof. 
Sepiolite fibrous structure consists of talc-like ribbons with two sheets 
of tetrahedral silica units, linked by means of oxygen atoms to a central 
sheet of magnesium, so that the tetrahedral sheet is continuous, but with 
the directions of the apical end of the silica tetrahedrons of the 
adjacent ribbons inverted. Each ribbon alternates with channels along the 
fibre axis. The octahedral cations which are present at the ribbon edges 
complete their coordination with water molecules (coordination water). 
Additional water molecules are linked by hydrogen bonds to the coordination 
water, both at the outer surfaces and into the channels (zeolitic water). 
Sepiolite has 8 octahedral cations for every half unit formula, usually 
occupied by 8 Mg.sup.2+ ions following a trioctahedral model. 
The tetrahedral sheet is mostly occupied by Si.sup.4+ iones; although 
tetrahedral Al is present at a rate of 0.04- 0.48 for every 12 tetrahedral 
sites. 
The charge deficiency is internally compensated for to a great extent, 
whereby the low values of the ion-exchange capability are mainly due to 
charge deficiencies on external surfaces. 
Sepiolite, therefore, has channels along the fibre giving the former the 
appearance of a wall which alternatingly lacks bricks. The channel section 
is in the range of 3.8.times.9.4 .ANG.. 
Sepiolite has a color ranging from dark grey to light cream depending on 
the nature of the impurities present in the mineral. The main outstanding 
color responsible factors are the Fe, Mn and organic matter contents. The 
interest in bleaching sepiolite is obvious for a wide range of uses, 
wherein sepiolite use is restrained only due to color reasons. Among such 
applications, its use in paints, detergents, rubber, paper, etc., could be 
pointed up. There are a great number of processes for clay bleaching, 
especially for kaolin. They all basically coincide in dispersing the clay 
into an acid or weakly acid medium, treating it then with a reducing 
agent, mainly with sodium hydrosulfite, also with the possibility of using 
complex-forming agents to prevent from metal readsorption, mainly of Fe, 
dissolved by this method. 
However, when trying to apply these processes to sepiolite bleaching, the 
results are negative, neither reduction, nor dissolution of the Fe.sup.3+ 
present in sepiolite being attained. On the contrary, due to the acid pH 
at which the treatment is conducted, a part of the Mg of the octahedral 
sheet of the sepiolite, appreciably more soluble than Fe, dissolves, due 
to the Mg bigger capability of polarization, resulting in the collapsing 
and the total or partial destruction of the sepiolite structure. 
Other clay-bleaching methods based on the use of electromagnetic 
separators, have not proved to be efficient in sepiolite bleaching. 
One could also alternatively think of process for removing organic matter 
for bleaching the mineral, since the former is partially responsible for 
color. The aim of the customarily used methods is the oxidation of said 
organic matter with oxidants such as hydrogen peroxide. When said 
processes are applied to sepiolite, not only a color improvement is not 
obtained, but it notoriously worsens. 
Such as it has already been pointed out, charge deficiencies of sepiolite 
are most compensated internally, wherefor it has a small ion-exchange 
capability, generally from 10 to 25 meq/100 g, mainly due to the charge 
deficiencies on external surfaces. Ion-exchange capability comes to be a 
highly interesting property of organic materials, as it makes their use 
possible in many applications. Thus, materials with high ion-exchange 
capabilities, such as zeolites, exchanger resins, clays, etc., are used 
for removing noxious cations in water purification; as cation sequestrants 
of detergents, for removing Ca.sup.2+ and Mg.sup.2+ cations which harden 
water and make washing difficult; for removing radioactive isotopes in 
sewage of nuclear plants, etc. 
Other application for which ion-exchange capability is highly interesting, 
is the use as catalytic support, as this property enables the preparation 
of catalysts by cation-exchange techniques, which makes it possible to 
modify, depending on the exchanged cation, the catalytic properties of the 
materials, such as; acidity, pore size, stability, etc. Sepiolite is 
widely used nowadays as nickel, molybdenum, wolfram and cobalt support in 
catalysts of petroliferous fraction catalytic hydrotreatments, mainly in 
demetallization, as nickel, palladium and rhodium support in hydrogenation 
catalysts and, finally, it is also used as an ingredient in cracking 
catalyst dye. 
Nowadays, said catalysts are usually prepared by impregnation techniques, 
less efficient than those of ion-exchange for multiple catalytic 
applications. For this reason, the increase in sepiolite ion-exchange 
capability would allow to increase sepiolite catalytic applications 
highly, mainly in the field of oil and its derivatives refining. 
Different treatments are reported in the bibliography of sepiolite with 
sodium hydroxide, either at the atmospheric pressure or at higher 
pressures, wherein an increase in the ion-exchange capability is obtained. 
These treatments display, however, the following disadvantages: 
In those treatments conducted at the atmospheric pressure, there is no 
dissolution of the organic matter, wherefor nor is it obtained an 
improvement in the mineral whiteness; on the contrary, there is a certain 
darkening of the mineral, probably due to the oxidation of Fe.sup.2+ into 
Fe.sup.3+. 
In those treatments conducted at the atmospheric pressure, very high NaOH 
concentrations are needed, over 6 eq/1. 
In those treatments conducted in an autoclave, under pressure, although 
lower NaOH concentrations (of about 2 eq/1) are needed and there is an 
improvement in the material whiteness, there is a dissolution of an 
important part of the silicon of the tetrahedral sheet of the sepiolite 
due to the excessively high values of pH and temperature. This results, on 
the one hand, in the obtention of a lower yield of the process, since a 
big part of the material dissolves in the reaction medium and, on the 
other, in a remarkable unstabilization of the material structure, which 
leads to a great loss of its crystallinity, a much more amorphous product 
with respect to the starting sepiolite being thereby obtained. This is a 
most important factor in a big number of catalytic applications, since 
crystallinity loss often involves notorious losses of catalytic activity 
and stability, as it happens, for instance, in the given case of cracking 
catalysts. 
For that reason, the processes known until now for improving sepiolite 
whiteness and ion-exchange capability have not met important industrial 
applications, since, either they do not improve any of these properties, 
or they result in a marked deterioration of the crystalline structure of 
the material. 
From all the above we could infer the interest of sepiolite bleaching and 
ion-exchange capability improvement for numerous applications as well as 
the fact that the methods usually used for clay bleaching come to be 
inefficient in the case of sepiolite, with important disadvantages in 
those methods developed for increasing ion-exchange capability, which has 
hindered up to now the use thereof on a large scale. 
DISCLOSURE OF THE INVENTION 
The present invention refers to a process for bleaching and increasing the 
ion-exchange capability of the sepiolite mineral, which overcomes the 
disadvantages of the prior art. 
For the process according to the present invention, sepiolite or sepiolite 
mineral can be used, understanding as such that clay mineral containing 
sepiolite as major component, being allowed to be accompanied by other 
clays, feldspars, quartz, micas, etc. In order to obtain whiter products, 
the starting sepiolite should have a reflection index over the 50%, 
preferably over 70%. 
Naturally, this reflection index can be lower than 50%, in case that it is 
the increase in the ion-exchange capability what is mainly intended to be 
achieved. 
The process according to the present invention has the purpose to obtain 
the bleaching and increase in the ion-exchange capability of sepiolite, 
basically consisting of treating sepiolite with an aqueous solution of a 
basic salt, preferably calcium carbonate, or mixtures of various basic 
salts, with possibility to additionally incorporate small amounts of 
alkaline or ammonium hydroxides. Said solution must have a basic salt 
concentration ranging from 0.1N to 10N, preferably from 0.5N to 6N, and a 
sepiolite concentration ranging from 1% to 50% by weight, preferably from 
3% to 20%. The amounts of hydroxides to be added should be such that their 
concentrations in the medium range from 0 to 0.5N, preferably from 0 to 
0.3N. 
Alternatively, instead of water, any other polar liquid can be used as 
reaction medium, such as ethylene glycol, acetone, methanol, ethanol, 
methylethylketone, isopropanol, etc. 
Treatment of sepiolite with said solution must be conducted at a high 
temperature, over 50.degree. C. and preferably over 100.degree. C. 
Likewise, said treatment should be preferably conducted in an autoclave, 
at a pressure over the atmospheric one, preferably between 2 and 50 
atmospheres. The treatment time is inversely proportional to the 
temperature used, being able to range from 1 minute to 8 days, preferably 
from 30 minutes to 6 hours, if the reaction is conducted under pressure in 
autoclave. 
When this treatment is over, the obtained dispersion is filtered and 
filtration waters are collected with a more or less dark color, depending 
on the nature of the sepiolite to be treated and on the more or less 
energetic character of the treatment; as well as a solid which, once it 
has been washed for several times with water or with any other polar 
liquid of those above mentioned, is dried by means of any conventional 
drying process: in a stove, in the environmental atmosphere, by an air 
flow, by lyophilization, under vacuum, in spray dryer, on fluidized bed, 
etc. 
Likewise, separation of the solid from the filtration waters can be carried 
out by other processes than filtration, such as decantation or 
centrifugation. So as to make solid separation easier, both from the 
reaction medium and from the washing waters, small amounts of flocculants 
or of different salts acting as such can be added. Among the different 
products which can be used for this purpose, the following can be quoted: 
organic polymers, sodium chloride, magnesium chloride, aluminium chloride, 
calcium chloride, aluminium nitrate, ferric chloride, ferric sulphate, 
calcium hydroxide, magnesium hydroxide, aluminium sulphate, aluminium 
polyhydroxides, aluminium polyhydroxychlorides, etc. 
The product obtained by the process according to the present invention 
shows a whiteness noticeably higher than that of the starting sepiolite, 
expressed as reflection index. In case one wishes to further improve the 
whiteness of this product, the treatment can be completed with a bleaching 
process of the conventional type, similar to that used for bleaching other 
clays, particularly kaolin, which are reported in the bibliography. For 
the sake of instance, a typical treatment of this type could consist of 
contacting the obtained material and an acid solution of a reducing agent, 
preferably sodium dithionite, with the possibility to add, besides, to the 
reaction medium, some organic polyfunctional water-soluble compound able 
to form a complex with iron. It should be noted that said treatment 
applied to natural sepiolite does not produce any bleaching of the 
starting material; whilst, when applied to sepiolite which has previously 
been bleached by the process disclosed in the present invention, there is 
a new additional increase in the material whiteness. 
During the treatment as described, different modifications of the chemical 
composition and of the sepiolite structure take place. First of all, there 
is a slight displacement of some of the picks of the X Ray diffractogram, 
which shows an increase, during treatment, in the spacing of some of the 
crystallographic planes corresponding to sepiolite. Table I shows the data 
corresponding to a typical X Ray diffractogram of the obtained product, as 
well as of the starting sepiolite and of the natural mineral called 
Loughlinite. As it can be seen, there is an excellent concordance between 
the data of the obtained material and those of Loughlinite, for which 
reason, sepiolite was transformed, presumably, during treatment into 
Loughlinite, this being a mineral which, on the other hand, is 
extraordinarily scarce in nature. 
TABLE I 
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X Ray Diffraction Data of the Obtained Material, of Starting 
Sepiolite and of Natural Loughlinite. 
Obtained Material 
Sepiolite Loughlinite 
Spacing Spacing Spacing 
(A) Intensity 
(A) Intensity 
(A) Intensity 
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12.9 100 12.1 100 12.9 100 
7,66 5 7.5 3 7.63 5 
-- -- 6.7 4 -- -- 
4.85 5 -- -- 4.81 5 
4.48 13 4.49 9 4.51 7 
4.36 18 4.29 14 4.34 18 
3.80 9 3.74 10 3.83 6 
3.60 9 -- -- 3.63 7 
3.32 5 3.34 14 3.31 5 
-- -- 3.18 7 -- -- 
______________________________________ 
As far as the chemical composition is concerned, Table II shows a typical 
chemical analysis of a material obtained according to the present process 
of the invention, compared with the sepiolite used for its preparation. It 
can be seen that there is a notorious increase in the amount of sodium 
present in the material, of about a 7%. This sodium is capable of being 
exchanged with other cations, thus being responsible for the increase in 
the ion-exchange capability. Said materials show very much high 
ion-exchange capabilities, in all cases over 100 meq/100 g, very much 
higher than the exchange capability of natural sepiolite. 
TABLE II 
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Typical Chemical Analysis of the Obtained Materials and of 
Starting Sepiolite. 
Obtained Material 
Sepiolite 
______________________________________ 
SiO.sub.2 59.0 63.5 
Al.sub.2 O.sub.3 
1.1 1.1 
MgO 21.5 23.4 
CaO 0.2 0.2 
Fe.sub.2 O.sub.3 
0.6 0.7 
Na.sub.2 O 7.1 0.3 
K.sub.2 O 0.2 0.4 
C.L.* 10.1 10.3 
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*Calcination Losses 
Comparison being made to the existing methods for improving the 
ion-exchange capability on the basis of the treatment with NaOH at high 
temperatures and pressures, a smaller loss of crystallinity of the 
starting sepiolite is observed, highly crystalline materials and a smaller 
dissolution of the silicon of the tetrahedral sheet of sepiolite being 
obtained. This becomes clear mainly by two facts: firstly, by the height 
of picks in the X Ray diffractogram, which is much higher in the materials 
treated according to the present process than in those treated with sodium 
hydroxide according to the prior art; and, secondly, by the analysis of 
the filtrate of the reaction medium, wherein low silicon concentrations 
are detected, ranging from 100 to 200 ppm, in the case of the process 
according to the present invention, whilst, when the treatment is 
conducted in similar conditions with NaOH, silicon concentration in the 
filtrate is over 2000 ppm. 
On the other hand, the process according to the present invention not only 
does not involve a higher cost with respect to the treatment of sepiolite 
with sodium hydroxide, but, as it is carried out with basic salts, 
preferably sodium carbonate, and as it is a much cheaper product than said 
hydroxide, it involves an important saving in the cost of the reagents 
used in the process. Besides that, as, according to the present process, 
there is a smaller dissolution of the silicon, the yield of the process is 
higher if compared with the alternative processes, as the loss of raw 
matter by dissolution in the reaction medium is prevented from. 
Among the basic salts which can be used according to the present process, 
the following can be cited: sodium carbonate, sodium bicarbonate, 
potassium carbonate, tetrasodium pyrophosphate, sodium acetate, etc. Such 
as it has already been mentioned, the pH of the reaction medium must be 
high for the dissolution of the organic matter to take place, for which 
small amounts of metal hydroxides of ammonium hydroxide can be added. 
Nevertheless, it is advisable to use minimum hydroxide concentrations, 
because, on the contrary, they would result in the dissolution of a big 
part of the silicon of the sepiolite tetrahedral sheet. 
In order to obtain optimum whiteness degrees, it is advisable for the 
mineral not to contact with the metal parts of the used machinery during 
all stages of the process and, particularly, during its grinding and 
preparation, since it could produce contaminations due to the mineral 
abrasion, which would damage the whiteness of the final products. Due to 
this reason, it is advisable to use ceramic machinery and mills, or 
machinery and mills coated with teflon or with any other protecting 
material, in order to avoid contact between sepiolite and metal.

WAYS OF EMBODIMENT OF THE INVENTION 
A series of examples illustrating the present invention are hereinafter 
detailed. Example 5, corresponding to a process for increasing the 
ion-exchange capability according to the prior art, has been included 
herein for comparison purposes. 
EXAMPLE 1 
Sepiolite mineral from Vallecas (Madrid) with a content in sepiolite of 
90%, with a whiteness expressed as reflection index of 67%, an 
ion-exchange capability of 15 meq/100 g, a X-Ray diffractogram 
corresponding to that shown in Table I and a chemical analysis expressed 
as metal oxides, as indicated ianTable III, was ground in a ceramic ball 
mill up to a particle size smaller than 74 um. 100 g of this mineral were 
then dispersed into 1 liter of a 6N CO.sub.3 Na.sub.2 solution. Dispersion 
was carried out by stirring for 5 minutes in a Cowles-type stirrer at 1500 
rpm. The thus obtained dispersion was introduced into an autoclave and 
treated for 4 hours at 200.degree. C. under autogenous pressure. The 
product was filtered, washed up to 6 times with deionized water and then 
dried in a forced air stove, at 100.degree. C. The obtained product has a 
reflection index of 74% and an ion-exchange capability of 143 meq/100 g, a 
X-Ray diffractogram such as that indicated in Table I; the rest of the 
characteristics appearing shown in Table III. 
EXAMPLE 2 
The starting material used in Example 1 was submitted to a treatment 
identical to that of said example with a 2N CO.sub.3 Na.sub.2 solution. 
Results are shown in Table III. 
EXAMPLE 3 
One hundred grams of the starting material of Example 1 were treated with a 
liter of 4N CO.sub.3 Na.sub.2 solution and 4 g of NaOH were added in an 
autoclave under similar conditions as those of Example 1. The obtained 
product shows a reflection index of 76% and an ion-exchange capability of 
196 meq/100 g. 
EXAMPLE 4 
Example 3 was repeated with addition of 5.6 g of KOH instead of NaOH, 
maintaining the CO.sub.3 Na.sub.2 solution concentration and the rest of 
the operation conditions. The obtained product, the characteristics of 
which are summarized in Table III, presents a reflection index of 75% and 
an ion exchange capability of 177 meq/100 g. 
EXAMPLE 5 
100 g of the starting sepiolite of Example 1 were dispersed into 1 liter of 
an aqueous solution containing 40 g NaOH. The dispersion was treated in 
autoclave at 190.degree. C. for 4 hours. The product, once filtered, 
washed and dried, presents the characteristics indicated in Table III. 
From the comparison of the results shown in Table II, it can be seen that 
the material obtained in Example 5 presents a whiteness and an 
ion-exchange capability similar to those of the other examples, but it has 
however undergone a greater dissolution of the tetrahedral silicon, as 
proved by the lower content in SiO.sub.2 detected in the chemical 
analysis. On the other hand, the X-Ray diffractogram obtained with this 
material, although it basically coincides with that indicated in Table I, 
presents a lower crystallinity with respect to the other materials, 
doubtlessly due to the dissolution of a bigger amount of Si having been 
produced in the present Example. 
EXAMPLE 6 
Ten grams of the material obtained in Example 1 are dispersed into a liter 
of a solution containing 0.5 g sodium hydrosulphite, 0.03 g potassium 
aluminium sulphate, 0.5 g mercaptoacetic acid, 7 g sodium oxalate and 7 g 
oxalic acid. This solution was previously degasified in ultrasound bath 
for 20 minutes. The dispersion is treated at 80.degree. C. for 60 minutes 
in a reactor provided with stirring. It is then filtered and washed with 
0.1N oxalic acid and further with deionized water until reaching a 
slightly alkaline pH. The reflection index of the obtained product, once 
it has been dried and ground comes to be 80%. 
With the application of this treatment to the starting sepiolite or 
sepiolite mineral, there would not be a noticeable improvement in the 
whiteness of the final product, because the process, according to the 
present invention, produces an alteration of the structure of the starting 
material which makes it capable of being submitted to the conventional 
clay-bleaching treatment with reducing agents. 
TABLE III 
______________________________________ 
Characteristics of the Obtained Materials. 
Ex- Ex- Ex- Ex- Ex- 
Natural amp. amp. amp. amp. amp. 
Sepiolite 1 2 3 4 5 
______________________________________ 
Solution 
CO.sub.3 Na.sub.2 
-- 6N 2N 4N 4N -- 
NaOH -- -- -- 0.1N -- 2N 
KOH -- -- -- -- 0.1N -- 
Reflec- 67 74 75 76 75 76 
tion Index 
(%) 
Ion-Ex- 16 153 146 186 162 132 
change 
Capability 
(meq/ 
100 g) 
Chemical 
Analysis 
(%) 
SiO.sub.2 
63.5 59.0 59.0 58.0 58.0 56.0 
Al.sub.2 O.sub.3 
1.1 1.1 1.5 1.3 1.3 1.3 
MgO 23.4 21.5 23.0 22.0 22.5 24.4 
CaO 0.2 0.2 0.3 0.2 0.2 0.3 
Fe.sub.2 O.sub.3 
0.7 0.6 0.5 0.4 0.6 0.5 
Na.sub.2 O.sub.3 
0.3 7.1 6.3 7.5 7.0 6.5 
K.sub.2 O 
0.4 0.2 0.2 0.1 0.4 0.1 
C.L.* 10.3 10.1 9.4 10.4 10.3 11.2 
TOTAL 99.9 99.8 100.2 99.6 100.3 100.3 
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*Calcination Losses at 1100.degree. C.