Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1999-07-01
2003-08-05
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S681000, C210S682000, C210S683000, C210S686000, C210S688000, C210S691000, C210S747300
Reexamination Certificate
active
06602421
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the purification of contaminated groundwater and, more specifically, to the purification of groundwater having hexavalent chromium.
BACKGROUND OF THE INVENTION
Recently, zero-valent iron filings have been identified as a suitable constituent for use in permeable reaction wall technology to remove certain contaminants from groundwater. This technology uses a form of passive groundwater remediation which typically involves placing a reactive wall or barrier in the flow path of contaminated groundwater. According to this technology, iron filings are typically mixed with sand to allow for a suitable permeability of the reactive barrier. The wall thickness and permeability are selected to provide for an adequate residence time of the contaminated groundwater within the barrier to reduce certain contaminants in the water to a desired level, such as below drinking water levels. The wall is typically oriented perpendicular to the flow path of the groundwater.
In some cases, a “funnel and gate” configuration may be used. The “funnel” consists of a sealable joint sheet pile or slurry which directs contaminated water to the iron wall or a “gate” and also prevents untreated groundwater from flowing around the gate. The impermeable funnels allow containment and treatment of a contaminant flow path (or plume) without constructing an iron wall across the plume's entire width.
An advantage of this type of groundwater remediation is that there are very few operating and maintenance costs after the reactive barrier is installed. Groundwater monitoring before and after installation of the reactive barrier is required in order to verify the effectiveness. Permeable barrier remediation technology is a growing field and is anticipated to be a major cost-effective groundwater remediation methodology of the future.
It has been reported that the present cost of zero-valent iron in a particle size suitable for a permeable reactive wall is approximately $400 per ton. If the volume of the wall is large, the cost of the reactive zero-valent iron can be considerable. Moreover, iron filings may have other elements, such as sulfur, selenium, arsenic, cadmium, lead, copper, and mercury, in a form which allows them to leach relatively easily to the environment. In addition, iron filings tend to absorb carbon dioxide from air or tend to be reactive with carbonate ions or carbonic acid from aqueous solution to form insoluble carbonates, thereby reducing the permeability by the plugging action of carbonates. Therefore, it is desirable to identify a constituent which might be used as a permeable barrier to remediate or purify contaminated groundwater. Preferably, such a constituent is not as costly as iron filings, does not contain certain elements in a readily leachable form, and does not absorb carbon dioxide from air or react with carbonate or carbonic acid.
SUMMARY OF THE INVENTION
In view of its purposes, the present invention provides a method for purifying contaminated water comprising passing contaminated water through a porous bed comprising steel slag. It has been found that the porous bed may consist solely of steel slag, with no other constituent or filler, such as sand, mixed with the steel slag. Preferably, the method involves first providing an underground barrier comprising a porous bed of steel slag in a flow path or plume of contaminated groundwater then allowing the groundwater to pass through the barrier to purify the groundwater. The slag can be used as formed, with no further grinding needed, and preferably is sieved such that the slag used has particles with a median diameter within the range of about one-eighth of an inch to one inch and preferably from about one-quarter of an inch to three-quarters of an inch. The present invention is particularly well-suited to removing hexavalent chromium from contaminated groundwater.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a method which utilizes steel slag as a permeable barrier within a flow path or plume of contaminated groundwater to purify the groundwater. Steel slag is the cooled, solidified co-product obtained in the process for making steel and is formed when steel-containing ores and/or scrap are treated and/or purified in a steel making furnace. In general, steel slag is itself a non-metallic product, consisting primarily of silicates and aluminosilicates of calcium and other bases that develop in a molten condition during steel making. The particular constituents and concentrations of those constituents vary depending on the process conditions of the steel furnace from which the slag is removed. Nonetheless, it is believed that any steel slag can be used in the present invention. Typically, steel slag has generally the following composition, although it varies as stated above:
Constituent
Weight %
SiO
2
9-20
CaO
30-59
Al
2
O
3
0.1-2.5
FeO
5-20
MgO
0.6-8
Sulfur
0.15-0.62
(Reported
as SO
3
)
MnO
1.3-10
TiO
2
0.4-0.9
P
2
O
5
1.5-2.3
Steel slag can also include other constituents not listed above such as Fe
2
O
3
, K
2
O, Na
2
O, and r
2
O
3
.
The median particle size of the steel slag should be within a particular range. For example, if the median particle size is too large, then the overall surface area available for reaction with the contaminants is decreased, thereby decreasing the efficiency of the permeable wall. On the other hand, if the median particle size is too small, such as less than about one-eighth of an inch, then the fine particles might tend to cementitiously harden when formed into a wall and thereby become relatively impermeable to groundwater. The formation of an impermeable wall would be severely detrimental to the purification system in that the contaminated groundwater would find another route around the barrier and therefore would remain contaminated.
The steel slag used for the permeable wall may have a fairly broad distribution in particle size and can include the presence of very fine particles and large particles. In the case of a broad distribution, the presence of large particles would tend to limit the formation of regions of cementitious hardening otherwise caused by the small particles. The use of steel slag having a broad distribution is desirable for the reason that such slag requires less processing (i.e., little or no separation) before use as a permeable wall. On the other hand, the steel slag used for the permeable wall may have a fairly narrow distribution in particle size. The use of steel slag having a narrow distribution is desirable for the reason that such slag tends to have a more homogenous reactivity throughout the permeable wall.
Generally, the steel slag used should have particles with a median diameter of about one-eighth of an inch to one inch, and preferably between about one-quarter of an inch and three-eighths of an inch. To obtain steel slag particles of this size, the formed steel slag need not be crushed or ground, but can merely be sieved in a known way to achieve a set of particles having an average diameter within these ranges. In some cases, steel slag as formed can be used.
It has been found that the steel slag generally has a sufficiently high permeability (e.g., about 4.5×10
−2
cm/sec) to be the sole constituent in the porous bed. Moreover, it has been found that steel slag retains this high permeability even after water has passed through it. As mentioned above, in iron filings technology, the iron filings are typically mixed with sand before placed in a plume of contaminated groundwater. Because the permeable wall of the present invention is entirely the reactive material, the efficiency of the wall, measured as reactive sites per unit weight of wall, is increased as compared to a wall of iron filings.
The method of the present invention is used for purifying contaminated gro
Cintins Ivars
RatnerPrestia
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