Chemistry of inorganic compounds – Zeolite
Reexamination Certificate
2001-03-16
2003-11-04
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
C423S709000, C423S716000, C423SDIG002, C423SDIG002
Reexamination Certificate
active
06641796
ABSTRACT:
TECHNICAL FIELD
This invention relates to zeolites, and especially zeolite A and mixtures of zeolite A and zeolite X having a small crystal size and particle size, and enhanced liquid carrying capacity, cation exchange rate, and cation exchange capacity. Such zeolites have a number of uses, but are especially useful as builders in combination with detergents in cleaning formulations.
BACKGROUND OF THE INVENTION
Zeolites, as is commonly known in the art, are crystalline aluminosilicates having fully cross-linked open framework structures made up of corner-sharing SiO
4
and AlO
4
tetrahedral groups. Zeolites belong to the class of minerals referred to generally as tectosilicates, because their crystalline architecture can be idealized as being constructed from silicon atoms in tetrahedral, four-fold coordination with oxygen atoms in a 3-dimensional lattice. Each silicon atom in the structure has a nominal 4
+
charge and shares 4 oxygen atoms (each having a nominal charge of 2
−
) with other silicon atoms in the crystal lattice.
Substitution of the isoelectronic Al
3+−
for Si
4+
creates a charge inbalance on the lattice that must be rectified by the incorporation of additional cations close by Al sites in the framework. Steric accommodation of these cations directs the crystallization of aluminosilicates towards the formation of more open structures containing continuous channels or micropores within the crystal. These structural micropores in the anhydrous zeolites allow the passage and adsorption of molecules based on size giving the materials molecular sieving properties. The cations themselves are not part of the crystal framework and can usually be replaced by equivalently charged species without damage to the lattice. In zeolite structures such as A and X the pore size is large enough to permit the facile passage and exchange of cations in aqueous solutions. The as-synthesized forms of zeolites A and X contain Na cations that can be exchanged for Ca
2+
and Mg
2+
ions present in so-called “hard” waters and this gives these two zeolites particular value as water “softening” builders in detergent formulations.
Zeolites in general can be represented empirically as:
M
2
O.Al
2
O
3
.xSiO
2
.yH
2
O;
wherein: M represents an exchangeable cation of valence n which is 1 or 2; x represents the number of moles of silica per mole of alumina and is typically about 2 for NaA and 2-3 for zeolite X; and y represents the number of moles of water per mole of alumina. M is typically a Group I or II ion, although other metal, non-metal and organic cations may also balance the negative charge created by the presence of aluminum in the structure. In addition to Si
4+
and Al
3+
, other elements can also be present in the zeolitic framework.
Zeolites are frequently categorized by their crystalline structure. See W. M. Meier, D. H. Olson, and C. Baerlocher,
Atlas of Zeolite Structure Types,
Elsevier Press (1996) 4
th
edition. Among these structure types are zeolite A and zeolite X, which are the subject of the present invention. Zeolite A has the usual formula of: Na
2
O.Al
2
O
3
.2.0SiO
2
.4.5H
2
O, and zeolite X has an empirical formula of: Na
2
O.Al
2
O
3
.xSiO
2
.6H
2
O, wherein x is in the range of 2-3.
The microporous structure makes zeolites useful in a number of industrial applications, such as drying agents molecular sieves (highly selective adsorbents), ion exchangers, and catalysts. Particles consisting of agglomerated zeolite crystals also have a macroporosity that is useful in the manufacture of dry laundry detergents, for example, where the particles act as a carrier for liquid detergent chemicals. The amount of liquid detergent chemical that can be carried by a particular zeolite powder is indicated by its liquid carrying capacity (LCC), often expressed as the grams liquid per 100 grams of “as-is” zeolite. Unless otherwise indicated herein, the “as is” weight of the zeolite includes any interstitial water of hydration. Zeolites for application as detergent builders are typically sold in a hydrated form wherein the weight of the hydrated zeolite is approximately 20-22% water, also referred to as 20-22% LOI. LOI stands for the “loss on ignition” resulting when a zeolite sample is heated to a specified elevated temperature to drive off volatile components such as water or organic materials.
The effectiveness of a detergent is often influenced in complex ways by the “hardness” of the water. Water hardness is measured in terms of the weight of CaCO
3
(in parts per million, ppm) equivalent to the concentration of soluble Ca and Mg present in water. Ca and Mg cations interfere with the action of the detergent in removing dirt from articles of clothing by reacting with detergent species. Ca in the dirt itself is thought to promote adhesion to fabrics and extraction of Ca by the zeolite may amplify the effectiveness of the detergent.
The Na-form of zeolite A exhibits a highly selective exchange affinity for Ca
2+
ions, the primary cation found in potable water in the United States, whereas zeolite X has a particularly high affinity for exchanging both calcium and magnesium ions. The greater facility with which the X phase takes up Mg
2+
is believed to be due to the larger pore size of this zeolite which more readily accommodates entry of the significantly larger hydrated Mg cation. When Mg is present in solution in high proportion it also interferes and slows the rate of Ca uptake by zeolite A. For waters containing predominantly Ca, zeolite A alone provides satisfactory exchange performance, but for waters containing higher proportions of Mg as well, it is advantageous to use combinations of zeolites A and X. In such applications it is preferable to use a zeolite X component of the so-called “low silica” variety (LSX) with a composition and exchange capacity per unit weight that is equivalent, or nearly so, to that of zeolite A. The separate manufacture of LSX for use in combination with zeolite A is more expensive, so it is advantageous to accomplish direct synthesis of the mixed zeolite Group I ion product in the same low cost process used to manufacture zeolite A.
To maximize the effectiveness of detergent components of a washing formulation, it is critically important to remove the hardness components from the wash water as rapidly as possible. Ca removal, or sequestration, by a solid material occurs via a sequence of steps: a) Ca
2+
diffusion through the solution to the zeolite particle; b) diffusion of Ca
2+
across the static film boundary at the crystal/solution interface; c) distribution of Ca
2+
over exchange sites by diffusion of the ion through zeolite micropores. The slowest, and therefore rate-determining, steps in this exchange process are believed to be associated with diffusion across the film boundary layer and distribution through the crystal. Vigorous agitation in the solution phase and dispersion of zeolite powder in the liquid facilitates transfer of Ca
2+
through the bulk solution so that this step is not rate-limiting. Recognizing this, zeolite manufacturers make every effort to manufacture zeolite detergent builder materials with smaller particle size so as to increase the net rate of Ca/Mg sequestration. Larger particles, especially those greater than 10 microns in diameter, must also be minimized to avoid the unsightly deposition of zeolite residues on dark colored articles of clothing. As would be expected, the rate of Ca removal from solution is strongly dependent upon the temperature of the exchange solution. Ca diffusion processes, and hence their exchange rates, in zeolites occur more rapidly in hot water than in cold. To be useful as performance builders in detergent powders for cold water application, it is desirable to improve the exchange rate of Ca on zeolite A powders.
One strategy to increase exchange rate is to significantly reduce zeolite particle size. For detergent zeolite powder, however, other criteria constrain the manufacture of very small zeolite par
Hinchey Richard J.
Micco Daniel J.
PQ Corporation
RatnerPrestia
Sample David
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