Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2001-04-13
2003-08-19
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S725000, C210S727000, C210S778000, C210S793000, C451S036000, C451S088000
Reexamination Certificate
active
06607670
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to methods and apparatus for separating water borne residuals created during the high pressure waterjet cutting of material in nuclear reactors and more particularly to a rapid process for continuously and rapidly removing same when using particulate matter in the Waterjet cutting such as garnet.
2. Description of the Prior Art
The waterjet cutting process when used in nuclear reactors uses an approximately 50,000 psi water stream which includes a small amount of particulate shot material such as garnet pellets to assist in the waterjet cut through the stainless steel or any other metal encountered in the reactor. Garnet is a form of silicate with traces of iron, aluminum as well as other possible trace elements such as chrome, magnesium, calcium, manganese and titanium. The typical chemical formula for garnet is: A
3
B
2
(SiO
4
)
3
, where A=iron, manganese, calcium or magnesium and B represents elements such as aluminum, iron, chromium or titanium. The waterjet garnet is substantially fractured by the high-pressure impact of the garnet on the cut stainless steel of the reactor. This results in a variety of particle sizes from less than 1 micron to about 300 microns.
Due to the high energy fracturing of the garnet during the waterjet cutting the creation of very small colloidal particles of garnet was seen. This is true for other particles used in Waterjet cutting such as stainless steel and others, although the amount of such created colloids may change. During this process Si(OH)
4
is created which, due to its chemistry (and the approximate neutral carrying water) the presence of charged hydroxyl complexes such as: (HO)
3
SiO are allowed. These bear a negative charge. This negative charge is a universal problem when particle removal from water is desired. Further, the creation of other typical very small colloidal particles of less than 10 microns that may have a variety of SiOx structures will also bear a negative net charge. Colloids are any particles, by definition, between about 0.005 and 10 microns in size, regardless of their chemical or biological composition.
A universal property of water borne colloidal silica, in neutral to near neutral pHs, is that they will bear a negative charge. This negative charge, combined with the small size creates a substantial particle removal problem.
Although colloidal particles are small, they have a very large surface area which permits the colloid to scatter light far beyond what might be suggested by its mass. The surface area value for the “typical particle” is in the range of 250-350 square meters per gram. This results in a very, very low quantity of colloidal silica causing significant turbidity (light scatter and failure to pass light) in any colloidally contaminated water. It has been found that the waterjet process water even when filtered through a 1 micro filter will still absorb over 98% of 400 nano-meter light at a distance of 5 feet in depth.
Another feature of the small mass of each colloid particle is that its electric charge (negative) to its mass is quite high. This high charge allows colloidal particles to repel other particles vigorously and, hence, hinders efficient removal or filtration or centrifuging (hydro cyclones, cyclone separators and centrifuges).
The negative charge is measured in a known manner using a Zeta potential meter. Typical waterjet cutting polluted reactor water was tested to have a value of between minus 20 and minus 40. An acceptable value for treated and filtered water is minus 5 to plus five as published by the Zeta meter manufacturer.
When “un-charge neutralized” colloidal particles are trapped in or on a filtering surface, the repulsion energy grows quickly. The net result is that the filtering surface is quickly “plugged” with a very low mass, but which mass also has a high repulsion energy to the acceptance of any more negatively charged mass in the water. Also, some colloidal particles, which remain uncharged or are neutralized, will not be filtered out at all, even with a 0.5 micro filter, and will pass into the filtrate causing turbidity.
When untreated colloidal particle filtration continues for a brief time, the bulk of the suspended mass in the water, which is non charged, cannot be efficiently filtered from the water due to premature filter plugging by the negatively charged colloidal particles. Experiments showed that process water that was even first passed through a cyclone separation continued to rapidly plug down field 3M filters of both 3 micron and 1 micron. This is due to the retained colloids and negative Zeta potential of the water.
The problem is further exasperated by the fact that the spent cutting water from the reactor also contains metal fines or what is referred to as swarf. None of these particles are colloidal. But, a very small amount is fractured by the waterjet process into a colloidal state. This occurs because about 0% of the suspended_stainless swarf, by weight, is fractured so vigorously into small particles that the chromium and nickel from the steel is forced into solution. Colloidal particles of metal are likely to be in a hydrated form similar to the hydrated/ionized silica and hence would contribute to the negative repulsion of the spent water stream.
This generation of fractured garnet and metal particles, during waterjet cutting in nuclear reactors that are less than 1 micron is size creates a huge problem in that prior art normal filtration is seriously hindered. In order to remove these very small particles, a very fine filter is required even though the overwhelming mass of the residuals are large enough to be trapped by a relatively high capacity corrugated depth filter such as a 2-10 micro fabric or a paper filter.
The very fine particles require fine filters, which by definition have a far lower capacity than a crasser filter. The use of fine filters leads to a very high generation of filter body waste, all of which will also be highly radioactive and thus extremely costly to dispose of.
The very fine, colloidal, particles further retard typical filtration because they are very negatively charged, repelling each other, leading to even a faster decline in filtering capacity and yet producing an even higher quantity of “dead” filters. This phenomena was confirmed through testing. During testing, fine filters were clogged in less than ten minutes with a fraction of the solids loading normally observed. Thus it was seen that the particle distribution for the whole body of total suspended waterjet particles in the process water, being from less than 1 micron to about 300 microns, is in a range that is totally unsuitable for mechanical separation with any level of efficiency.
It should be noted that during our testing, untreated process water was subjected to both a Krebs hydro cyclone and a Lykos liquid solids separator. Neither method could provide a removal efficiency, on average, of even 50%. This was due to the small negatively charged particles and the large negative Zeta potential of the water. Treatment of the waste water prior to mechanical separator treatment indicated some potential in substantially improving this mechanical method, but with treatment costs included, other methods of residual removal indicated much less expensive potential.
All the mentioned treatment systems were slow and inefficient and totally inappropriate to meet the needs for rapid removal of the pollutants within less than a minute and preferably within seconds.
In order to rapidly remove the waterjet residuals from the process water within the mentioned time factors, some form of traditional pre-filtration chemical treatment seemed to be required.
Properly treated nuclear reactor water needs a treatment and collection system, which will rapidly (within seconds) achieve the following required points:
a bulk solids in water separation efficiency of 98%+
the addition of any treatment solids requires minimization
high radioactivity due to the metal swarf ne
Baldwin Philip N.
Beatty Raymond E.
Day James E.
Guy Gary J.
Framatome Anp. Inc.
Hruskoci Peter A.
Matas Vytas R.
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