Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1999-12-08
2001-08-14
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
Reexamination Certificate
active
06274112
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a continuous process for preparing silica-based microgels whereby consistent quality of the microgel can be achieved with varying production rates.
DESCRIPTION OF THE RELATED ART
Polysilicate microgels (i.e., aqueous solutions formed by the partial gelation of a soluble silica) are well known in the art. These microgels can be prepared by partial gelation of an alkali metal silicate by mixing the silicate with a gel initiator, aging the mixture for a short time, and then stopping further gelation by diluting the mixture. Gel initiators are also referred to as “neutralizing agents” and/or “activating agents”. Mineral acids and alum are the most commonly employed gel initiators. Resulting microgels have commercial utility as a drainage and retention aid in paper making, as a flocculation agent in potable water purification plants, and in similar applications.
Several practical factors currently limit commercial use of polysilicate microgels, although they are excellent flocculents and environmentally benign. For example, microgel solutions are necessarily dilute, making it impractical to ship large volumes long distances. Therefore, microgels are typically produced at a field site by the user. Microgels also are prone to gel and to form silica deposits in equipment used to prepare the product. These problems can be overcome by equipment design and trained personnel in a factory environment, but present greater difficulty in field applications where the equipment should be relatively easy to operate and maintain.
Batch and continuous processes have been developed through the years to produce silica microgels. However, consistency in product performance has been found to vary considerably from batch-to-batch and even over relatively short periods of time with a continuous process.
Performance of silica microgels as flocculents has been shown to be highly dependent upon growing the silica microgels to the proper size before use. Two of the most important factors that affect the size of the microgel produced are pH during partial gelation of the silicate and reaction time, which includes mixing time, but is primarily aging time, or time until dilution of the silica microgel solution.
During partial gelation of the silica, pH is difficult to control when using a strong acid, such as sulfuric acid, as the gel initiator because typically about 85% of the silicate alkalinity is neutralized so the microgel can be used quickly after preparation. Small changes in the amount of initiator result in large variations in pH, which in turn result in changes in the microgel size and in microgel performance.
In a batch reactor, reaction time can be easily changed. However in a continuous reactor, reaction time is determined by the flow rate of the reactants and the volume of the reactor. If the flow rates of the reactants are changed, for example, to meet a change in demand rate for the silica microgel, then reaction time, size of the microgel formed, and therefore, microgel performance, will vary.
Moffett and Rushmere, in U.S. Pat. Nos. 5,279,807; 5,503,820; 5,648,055; and 5,853,616 disclose improved continuous processes for preparing polysilicate microgels wherein silica deposition is greatly reduced by mixing a soluble silicate solution and a gel initiator under specific conditions.
Moffett, Simmons, and Jones in U.S. patent application Ser. No. 09/119,468, filed Jul. 20, 1998 disclose a continuous process for preparing polysilicate microgels wherein elastically deformable vessels are incorporated into the process. Such vessels enable dislodging of deposits formed on vessel walls.
While the designs taught in these patents result in much decreased deposition, and have found commercial utility, there remains lack of consistent quality when production rate of microgels is varied. When flow rates of reactants are changed to meet varying demand production rates, aging time changes and hence, size of the microgel formed and performance can vary. Thus, the user must accommodate for varying performance.
A continuous process that could reliably produce polysilicate microgels of consistent quality having consistent performance at different production rates in the same equipment to meet varying customer demands would have high utility.
SUMMARY OF THE INVENTION
The present invention provides a continuous process for preparing polysilicate microgels comprising:
(a) contacting a feed stream comprising a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a water soluble silicate, a colloidal silica sol, and combinations thereof, with a feed stream comprising carbon dioxide in a contacting vessel to produce a mixture; and
(b) aging the mixture in an aging vessel to partially gel the mixture to produce an aged mixture,
wherein the contacting step, the aging step, or both, are performed at a pressure of at least about 172 kPa (about 25 psig).
The stream comprising carbon dioxide may contain free carbon dioxide, typically in the form of a gas or liquid, or in the form of a material that will release carbon dioxide under reaction conditions. Mixtures of carbon dioxide with other components are also contemplated.
The present invention and its particular embodiments provide advantages in a continuous process for preparing silica microgels, which include better pH control during the step where the feed streams are contacted; more consistent microgel size and performance; elimination of strong mineral acids in the process, which provides safety benefits as well as lower equipment costs; improved ability to remove silica deposits; and efficient system purge in the event water supply is lost.
DETAILED DESCRIPTION OF THE INVENTION
Polysilicate microgels are aqueous solutions formed by the partial gelation of a silica source, for example, a water soluble silicate, a colloidal silica sol, or combinations thereof.
Water soluble silicates include alkali metal silicates and polysilicates, such as sodium silicate, having in its most common form one part Na
2
O to 3.3 parts SiO
2
by weight. Microgels formed from soluble silicates typically are composed of water and linked silica particles having a diameter of 1 to 5 nm and a surface area of at least 500 m
2
/g, more typically of at least 1000 m
2
/g. The particles are linked together during preparation (i.e., during partial gelation) to form aggregates having three-dimensional networks and chains. Preferably, the silica source is an aqueous solution of a water soluble silicate.
Colloidal silica sols are commercially available, for example, from E. I. duPont de Nemours and Company, Inc., sold under the name Ludox® Colloidal Silica. Silica sols useful in this invention are composed of water and discreet silica particles having a diameter of 4 to 60 nm, preferably less than 50 nm. The sol particles also link together during partial gelation to form aggregates having three-dimensional networks and chains. Microgels formed from silica sols will typically have a surface area in the range of about 50 to 750 m
2
/g.
At a pH below 5, polysilicate microgels sometimes are referred to as polysilicic acid microgels. As the pH value is raised, these products can contain mixtures of polysilicic acid and polysilicate microgels, the ratio being pH-dependent. As used herein, the term “polysilicate microgel” or “silica microgel” includes such mixtures of polysilicic acid and polysilicate microgels.
Polysilicate microgels frequently are modified by incorporating aluminate ions into their structure. The aluminum may be present throughout the polysilicate aggregates, or only on their surface, depending on where the aluminum source is added to the process. Aluminum may be added to increase the rate of microgel formation, and thus to decrease the aging time. Aluminum added as aluminate also allows the microgel to retain its charge at low pH conditions. Silica sols may have aluminum incorporated in the sol particles. As used herein, the term “polysilicate microgel” or “silica microgel” includes poly
Moffett Robert Harvey
Simmons Walter John
E. I. Du Pont de Nemours and Company
Griffin Steven P.
Johnson Edward M.
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