Colloid systems and wetting agents; subcombinations thereof; pro – Continuous liquid or supercritical phase: colloid systems;... – Aqueous continuous liquid phase and discontinuous phase...
Reexamination Certificate
2000-11-15
2002-12-31
Metzmaier, Daniel S. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous liquid or supercritical phase: colloid systems;...
Aqueous continuous liquid phase and discontinuous phase...
C516S084000, C516S087000, C423S339000, C051S308000
Reexamination Certificate
active
06500870
ABSTRACT:
The present invention relates to a method for manufacturing of high purity silica sols. More specifically, the invention relates to a method of manufacturing of high purity silica sols involving use of phosphonic acid-based complexing agents.
BACKGROUND OF THE INVENTION
Silica sols have been known for decades and are nowadays used in a great variety of applications such as in the paper production, coatings, catalysts, anti-skid products, polishing agents for various industries including the electronic industry, especially for wafer polishing. In many applications, the use of silica sols contaminated with traces of transition metals, alkali and alkaline earth metals, aluminum and the like have often caused problems. The presence of these metals, also in small amounts, interfere which leads to poorly performing silica sols.
Presence of e.g. contaminating metals like alkali metals such as Na, K, alkaline earth metals like Ca and Mg, transition metals such as Fe, Cu, Mn, Ni, Zn and the like are harmful when present in silica sols. Especially, aluminum is known to cause severe problems when present in the silica sol.
It has thus been elaborated various methods to prepare silica sols, having low contents of the mentioned contaminating metals.
Previous methods to produce silica sols have involved treatment of silicic acid solutions, derived from alkali metal silicates by use of various types of complexing agents such as EDTA and oxalic acid to reduce the contents of the above-mentioned disturbing metal cations.
EP-B1-464 289 discloses a method for preparation of a purified silica sol. In this method, an alkali metal silicate solution is first diluted in water, then exposed to a strong cation exchange resin to remove sodium ions from a sodium silicate solution thus forming a silicic acid solution to which oxalic acid is added. The oxalic acid is believed to form anionic complexes with metal ions, especially iron ions in the silicic acid solution. These complexes can be removed from the solution by means of an anion exchange step. Yet another cationic exchange step is required to remove remaining metal cations from the solution. The silicic acid solution is then subjected to polymerisation to form a silica sol.
The method according to EP-B1-464 289 yields some reduction in metal cations, e.g. Al, Fe, in the silicic acid solution. One drawback with the method according to EP-B1-464 289 is that a strong anionic resin must be used to separate the oxalate metal complexes. This will require regeneration of the anion exchange resin before it can be reused. Furthermore, multiple exchange steps increase the risk of contamination.
Manufacturing of high purity silica sols from fused silica or silicon metal is also known in the art. Fused silica and silicon metal are, however, highly priced raw materials which make the production very expensive.
The present invention provides for an inexpensive and effective way to produce high purity silica.
SUMMARY OF THE INVENTION
The invention is a method for producing high purity silica sols using a phosphonic acid-based complexing agent, and the high purity silica sols produced therefrom.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a new method for manufacturing of high purity silica sols has been provided. The new method involves treating a silicic acid-containing solution by adding a phosphonic acid-based complexing agent to the solution, forming a precipitate of complexes of phosphonic acid-based complexing agent and metal cations, removing said complexes whereafter the silicic acid can be subjected to polymerisation. The invention is further defined in the appended claims.
More specifically, the method for the manufacturing of high purity silica sols from a silicic acid-containing solution comprises the following steps:
(a) adding a phosphonic acid-based complexing agent to the silicic acid-containing solution forming complexes with metal cations present in the solution;
(b) forming in said silicic acid-containing solution a precipitation containing phosphonic acid-based complexing agent and metal cations;
(c) removing the precipitation from the solution; and,
(d) polymerising the silicic acid in the solution to obtain a silica sol.
The silicic acid-containing solution of step (a), has a SiO
2
-concentration of from about 1 to about 20%, preferably containing from about 4 to about 10 weight % SiO
2
, most preferably from about 5 to about 7 weight % SiO
2
and having a pH of from about 1 to about 4.5, preferably from about 1.5 to about 4, most preferably from about 2 to about 3, can be prepared in any known way from any alkali metal silicate including potassium silicate, sodium silicate, and lithium silicate or mixtures thereof. The alkali metal silicates are well known in the art of silica sol manufacturing and are available in various grades. Preferably, the alkali metal silicate used for production of silica sol will be sodium silicate which will be further described herebelow. In case other alkali metal silicates will be used, the same processing steps will be applicable.
Suitably, sodium silicates have a SiO
2
:Na
2
O molar ratio of from about 1 to about 5, preferably from about 3 to about 4. The silicic acid-containing solution can be obtained by increasing the SiO
2
:Na
2
O molar ratio of the silicate solution. This can be performed by cation exchange, as described in The Chemistry of Silica, Iler, R. 1979 pages 333-334, preferably by contacting a sodium silicate-containing solution with a strong cation exchange resin to yield a silicic acid-containing solution. The silicic acid is presumed to comprise an aqueous solution of one or several units of Si(OH)
4
linked together.
Before the cation exchange step is performed, the sodium silicate solution is suitably diluted. Normally, the cation exchange step will dilute the alkali metal silicate solution further as water can be added during the cation exchange treatment thus yielding a lower SiO
2
-concentration.
Phosphonic acid-based complexing agents added in step (a) refer to phosphonic acids and salts thereof. The phosphonic acid-based complexing agents are capable of forming complexes with harmful metal cations, e.g. Al, Ca, Cr, Fe, Ti, Zr present as impurities in the silicic acid-containing solution.
Any phosphonic acid-based complexing agents soluble in the silicic acid-containing solution can be used as a complexing agent.
The phosphonic acid-based complexing agent may be added to the silicic acid-containing solution in acidic form, as a salt, or a mixture of salts.
Examples of useful phosphonic acid-based complexing agents include ethylene diamine tetra (methylene phosphonic acid), hexamethylene diamine tetra(methylene phosphonic acid), diethylene triamine penta(methylene phosphonic acid), 2-phosphonobutan-1,2,3-tricarboxylic acid, propylene diamine tetra methylene phosphonic acid)(N,N,N′,N′-tetra(phosphonomethyl)-1,2-diaminopropane(PDTP), 2-hydroxyethyl iminobis(methylene phosphonic acid), 2-ethylhexyl iminobis(methylene phosphonic acid), n-oktyliminobis(methylene phosphonic acid), cyclohexane-1,2-diaminetetrakis(methylene phosphonic acid), cyclohexane,1,2-diaminetetrakis(methylene phosphonic acid), pentaethylenehexamineoctakis(methylene phosphonic acid), N,N-bis(3-aminopropyl)amine hexakis(methylene phosphonic acid), glycine-N,N-di(methylene phosphonic acid), N-(2-hydroxyethyl)-N,N-di(methylene phosphonic acid), aminotris(methylene phosphonic acid), and acetodiphosphonic acid, and salts thereof.
Preferably, aminotris(methylene phosphonic acid)(ATMP) (also named nitrilotris(methylene phosphonic acid)) and (acetodiphosphonic acid(ADPA) (also named 1-hydroxyethane 1,1-diphosphonic acid) or mixtures of these phosphonic acid-based complexing agents are added to the silicic acid-containing solution.
It has been found that the formation of complexes and precipitation according to step (b) normally is optimal at a pH from about 1.2 to about 2.2, preferably from about 1.6 to about 2, most preferably from about 1.7 to about 1.9. Normally, the suitable pH valu
Larsson Kenneth Olof
Linsten Magnus Olof
Tokarz Bozena Stanislawa
Akzo Nobel N.V.
Metzmaier Daniel S.
Parker Lainie E.
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