Silica precipitate

Details Silica precipitate Silica precipitate

2000-08-30

2001-11-06

Therkorn, Ernest G. (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S639000, C210S651000, C210S652000, C210S728000, C210S734000, C210S735000, C106S481000, C423S339000

Reexamination Certificate

active

06312601

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the high flow treatment and purification of wastewater containing silica. More particularly, the present invention relates to process and apparatus for removing silica from large quantities of wastewater using a combination of filter membranes and organic polymers.
BACKGROUND OF INVENTION
Many industrial operations generate large quantities of water containing silica. For instance, chemical mechanical polishing (CMP) processes, widely used in the manufacture of semiconductor devices, produce waste water streams containing high quantities of silica. CMP processes are used to polish the silicon-based wafer surface during various stages of semiconductor manufacture. Waste streams containing the polishing slurry and silica are produced during CNP. The silica must be removed before the water can be safely discharged to the environment or recycled within the facility.
Dissolved silica in industrial cooling water is a major problem. Silica is a scale forming material commonly found in cooling water which can foul heat exchangers, pipes, valves, pumps, and boilers. No known inhibitor, chelating agent or dispersant exists which will significantly control silica's tendency to form scale. When the silica concentration in a cooling water system exceeds its solubility limit of roughly about 150 to about 200 milligrams per liter, silica polymerizes to form scale. It may also react with multivalent cations, such as magnesium and calcium, to form scale.
Researchers have examined many different methods of removing soluble silica, including the use of ferric sulfate, calcium chloride, magnesium chloride, magnesium sulfate, magnesium oxide, aluminum hydroxide, sodium aluminate and activated alumina. Activated alumina has received much attention in processes for removing silica. See, U.S. Pat. No. 4,276,180 to Matson and U.S. Pat. No. 5,512,181 to Matchett. Other aluminum containing compounds such as sodium aluminate, aluminum sulfate, and aluminum chloride in an alkaline environment (pH greater than 8) have been used to remove soluble and colloidal silica. See, U.S. Pat. No. 5,453,206 to Browne. However, these processes are not capable of processing large volumes of wastewater through high flow mechanical systems because of degradation of particles and particulates below 5 micron in size.
Microfiltration systems have been considered to remove silica contaminants from wastewater. However, traditional microfiltration membranes having a pore size of about 0.5 microns rapidly clog with silica that was precipitated with conventional inorganic coagulants. Such particulates are consistently less than 1.0 micron in size. Moreover, the inorganic coagulants cannot aid in the precipitation of microfine colloidal silica. The partially formed floc will also deform and block the membrane pores, preventing flow.
There is, therefore, a need in the art for improved processes for removing silica from wastewater.
Such processes and systems are disclosed and claimed herein.
SUMMARY OF THE INVENTION
The present invention is directed to a process for removing silica from large volumes of wastewater. In the process, a wastewater stream containing silica is treated with an organic polymer. The coagulant reacts with the silica to form spherical particulates which agglomerate into clusters having a size greater than about 5&mgr;. As used herein, a wastewater stream includes raw water containing silica as well as process water streams containing silica. organic and polymeric coagulants which can be used to achieve the desired particulate formation, such as polyacrylamides (cationic, nonionic, and anionic), epi-dma's (epichlorohydrin/dimethylamine polymers), DADMAC's (polydiallydimethylammonium chlorides), copolymers of acrylamide and DADMAC, natural guar, etc. The stoichiometric ratio of coagulant to silica is preferably optimized to result in acceptable silica removal at minimum coagulant cost. The required coagulant concentration will depend on several factors, including silica contaminant influent concentration, wastewater flow rate, silica contaminant effluent compliance requirement, coagulant/contaminant reaction kinetics, etc. For silica contaminants, the ratio of silicon to coagulant contaminant is typically in the range from 20:1 to 50:1, depending on the system, an preferably about 40:1. If small amounts of silica can remain in the effluent stream, then the ratio of silicon to coagulant can be 120:1 or even higher. The optimum mole ratio will also vary depending on the coagulant used. For instance, low molecular weight epi-dma and very high molecular weight epi-dma require from 3 to 5 times the dose to flocculate the silicon.
It has been found that the foregoing organic coagulants cause the silica to form well defined spherical particles having a typical particle size in the range from about 10&mgr; to 90&mgr;. The silica particles are easily separated from micro-filtration membranes enabling efficient silica removal without membrane degradation.
Small amounts of a supplemental coagulant can optionally be used in combination with the organic and polymeric coagulant to optimize the silica removal. Examples of typical supplemental coagulants include, aluminum chlorohydrate (“ACH,” Al
n
OH
2n−m
Cl
m
, e.g., Al
4
OH
6
Cl
2
with a typical Al:Cl ratio of 2:1), sodium aluminate (NaAlO
2
), aluminum chloride (AlCl
3
), and polyaluminum chloride (“PAC,” Al
6
OCl
5
). The typical mole ratio of silica to inorganic coagulant is about 25:1.
Treated wastewater is passed through a microfiltration membrane which physically separates the silica contaminant from the wastewater. Suitable microfiltration membranes are commercially available from manufacturers such as W.L. Gore, Koch, and National Filter Media (Salt Lake City, Utah). For instance, one GOR-TEX® membrane used in the present invention is made of polypropylene felt with a sprayed coating of teflon. The teflon coating is intended to promote water passage through the membrane. Such microfiltration membrane material has been found to be useful for many wastewater treatment systems. However, when used in a system for removing fluoride or silica, it has been observed that the coagulated particles adhere to the exterior and interior surface and plug the membrane. Backflushing was not effective in such cases.
The microfiltration membranes are used in a tubular “sock” configuration to maximize surface area. The membrane sock is placed over a slotted tube to prevent the sock from collapsing during use. A net material is placed between the membrane sock and the slotted tube to facilitate flow between the membrane and the slots in the tube. In order to achieve the extremely high volume flow rates, a large number of membrane modules, each containing a number of individual filter socks, are used.
The microfiltration membranes preferably have a pore size in the range from 0.5 micron to 5 micron, and preferably from 0.5 micron to 1.0 micron. By controlling the ratio of coagulant to silica contaminant, 99.99% of the precipitated contaminant particles can be greater than 5 microns. This allows the use of larger pore size microfiltration membranes. It has been found that the treated wastewater flow rate through 0.5 to 1 micron microfiitration membranes can be in the range from 150 gallons per square foot of membrane per day (“GFD”) to 600 GFD.
Solids are preferably removed from the membrane surface by periodically backflushing the microfiltration membranes and draining the filtration vessel within which the membranes are located. The periodic,. short duration back flush removes any buildup of contaminants from the walls of the microfiltration membrane socks. The dislodged solid material within the filtration vessel is flushed into a holding tank for further processing of the solids.
The wastewater treatment system disclosed herein is designed to provide compliance with the contaminant silica discharge effluent limits. Wastewater pretreatment chemistry creates insoluble silica contaminant particulates which are

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