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Zero valent iron/iron oxide mineral/ferrous iron composite for treatment of a contaminate fluid

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Zero valent iron/iron oxide mineral/ferrous iron composite for treatment of a contaminate fluid


The present inventors have discovered a novel composition, method of making the composition, system, process for treating a fluid containing a contaminant. The fluid may be aqueous. The contaminated fluid may be in the form of a suspension. The treatment reduces the concentration of the contaminant. The reduction in concentration of a contaminant may be sufficient so as to effect remediation of the fluid with respect to the contaminant. The treatment may reduce the concentration of a plurality of contaminants. The present composition, system, and process are robust and flexible. The composition includes zero valent iron, an iron oxide mineral, and ferrous iron. The ferrous iron promotes maintenance of the iron oxide mineral. The iron oxide mineral promotes the activity of the zero valent iron. The process and system may involve multiple stages. A stage may be optimized for treatment with respect to a particular contaminant. The present composition, system, and process are effective for treating a fluid containing one or more of a variety of contaminants such as toxic metals, metalloids, oxyanions, and dissolved silica. It may be applied to treating various aqueous fluids, such as groundwater, subsurface water, and aqueous industrial waste streams.
Related Terms: Iron Oxide

Browse recent The Texas A&m University System patents - College Station, TX, US
Inventor: Yongheng Huang
USPTO Applicaton #: #20120273431 - Class: 210719 (USPTO) - 11/01/12 - Class 210 
Liquid Purification Or Separation > Processes >Making An Insoluble Substance Or Accreting Suspended Constituents >Including Chemical Reduction

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The Patent Description & Claims data below is from USPTO Patent Application 20120273431, Zero valent iron/iron oxide mineral/ferrous iron composite for treatment of a contaminate fluid.

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BACKGROUND

Wastewater treatment is one of the most important and challenging environmental problems associated with coal-based power generation.

Using wet scrubbers to clean flue gas is becoming more popular worldwide in the electrical power industry. In the coming years, hundreds of wet scrubbers will be installed in the US alone. While wet scrubbers can greatly reduce air pollution, toxic metals in the resulting wastewater present a major environmental problem. The industry prepares to invest billions of dollars in the next decade to meet more the ever more stringent environmental regulations; unfortunately, a cost-effective and reliable technology capable of treating such complicated wastewater is still not available.

The compositions of FGD wastewaters vary greatly, depending not only on the types of coal and limestone used but also on the types of scrubber and processes used. Pretreatment method and management practices also affect wastewater characteristics. According to a recent survey by EPR1 (2006), untreated raw FGD wastewater could have TSS in ˜10,000 mg/L but after settlement, it falls to ˜10 mg/L; the pH typically falls in 5.8-7.3; sulfate is in the range of 1,000-6,000 mg/L; nitrate-N at level of 50 mg/L is not uncommon; chloride, alkalinity and acidity vary from hundreds to thousands ppm; selenium exists in various forms, ranging from dozens of ppb to over 5 ppm, among which, selenate could account for about half of total Se; arsenic ranges from a few ppb to hundreds of ppb; mercury ranges from below 1 ppb to dozens of ppb; and boron can be as high as hundreds of ppm.

Treatment of selanate-Se in wastewater is often considered to be one of the most difficult in toxic metal treatments. Selenium is a naturally occurring chemical element in rocks, soils and natural waters. Although Se is an essential micronutrient for plants and animals, it can be toxic at elevated levels and some of Se species may be carcinogenic. The hexavalent selenium is stable in oxic environments and exists as the selenate (SeO42−) anion, which is weakly sorbed by mineral materials and generally soluble. Tetravalent Se is the stable valence state under mildly reducing or anoxic condition (0.26 V<Eh<0.55 V at pH 7). It exists as the selenite (SeO32−) anion, which tends to be bound onto mineral surfaces (e.g., Fe and Mn oxides). Selenate and selenite are more toxic due to their high bioavailability than elemental selenium or metallic selenides.

A biological treatment system, ABMet, has been patented and is being marketed by GE Water.

However, there remains a need for a cost-effective and reliable treatment process for removing toxic pollutants from the wastewater generated by the wet scrubbers operated for flue gas desulfurization in coal-fired power plants.

SUMMARY

The present inventor has developed a chemical treatment process that can cost-effectively treat all major pollutants in the flue gas desulfurization (FGD) wastewater in a single process.

The present inventor developed a fluidized reacting system using a hybrid reactive solid/secondary reagent reactor that can cost-effectively remove many toxic metals from wastewater. The system and process are effective to treat an aqueous suspension. The system uses a reactive solid and a secondary reagent as reactive agents to rapidly reduce selenate to become insoluble selenium species, which are then adsorbed or precipitated along with various of other toxic metals (such as As and Hg, if present) in wastewater onto the iron oxide sludge. The system is particularly effective for removing selenate-Se.

The present process is effective for removing almost all concern toxic metals in an aqueous suspension; in addition, it can remove oxyanion pollutants and metalloids. More particularly, contaminants removable by the present system and process are: most toxic metals such as arsenic, mercury, selenium, cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum, and the like; metalloid pollutants such as boron and the like; many oxyanion pollutants, such as nitrate, bromate, iodate, and periodate, and the like; and the like.

The present system and process use common, non-toxic, and inexpensive chemicals. The present chemical treatment system costs much less to construct and operate than biological treatment systems, which tend to be more complex.

The present system and process are versatile and flexible. The present system and process are more robust and manageable than a biological process when exposed to toxic chemicals or any disturbances and changes in wastewater quality and quantity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a single-stage fluidized bed reactor;

FIG. 2 is a flow chart illustrating a three-stage reaction system;

FIG. 3 is a schematic illustrating a single-stage fluidized bed ZVI/FeOx/Fe(II);

FIGS. 4A, 4B are pictures illustrating a bench scale single-stage reactor;

FIGS. 5A, 5B are pictures illustrating an alternative bench scale single-stage reactor;

FIGS. 6A. 6B, 6C are pictures illustrating a bench scale three-stage ZVI/FeOx/Fe(II) fluidized-bed reactor system; and

FIG. 7 shows three panels of pictures illustrating settling of a mixture of Fe0 and magnetite powder rich of surface bound Fe(II); pictures taken after settling for 1 min (left panel), 3 min (middle), and 6 min (right).

DETAILED DESCRIPTION

The present inventors have discovered a novel system for treating wastewater. Experiments have demonstrated the system operable for removal of selenium present as selenate.

According to some embodiments, a reactor system includes zero valent iron. According to some embodiments, ferrous iron is added to a reactor system. The present inventor believes that ferrous iron acts as a passivation reversal agent for zero valent iron. The mechanism is complex. While not wishing to be limited by theory, the present inventor believes that passivation is partially caused by corrosion of iron in a water environment. The present inventor believes that ferrous iron acts to cause conversion of iron corrosion product on the surface of the zero valent iron to magnetite. According to some embodiments, a sufficient amount of magnetite is produced so as to optimize removal of toxic materials by a reaction system including zero valent iron. According to some embodiments, the process produces removable solids. According to some embodiments, the removable solids contain toxic material encapsulated in magnetite. According to some embodiments, the encapsulated toxic material is solid.

Thus, according to some embodiments, the process uses a highly reactive mixture of zerovalent iron (Fe0), iron oxide minerals (FeOx), and ferrous iron (FeII) to react with, absorb, and precipitate various toxic metals and metalloids from wastewater, forming chemically inert and well crystallized magnetite (Fe3O4) particles that can be separated from water and disposed with encapsulated pollutants.

According to some embodiments, the reactive zone is maintained near neutral pH.

The present inventor believes that boron in the wastewater further contributes to passivation and that ferrous iron removes boron form the zero valent iron.

It will be understood that wastewater is illustrative of an aqueous suspension. For example, the present inventor contemplates treating oil refinery waste. Further, the present inventor contemplates treating wetlands.

It will be understood that selenium is illustrative of a toxic material. Other common toxic materials are contemplated. For example, the present inventor contemplates removing arsenic, mercury, cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum, and the like; metalloid pollutants such as boron and the like; many oxyanion pollutants, such as nitrate, bromate, iodate, and periodate, and the like; and the like.

It will be understood that ferrous iron is illustrative of a secondary reagent. The secondary reagent is desirable adapted to act as a passivation reversal agent. Passivation is generally the process of rendering an active material, for example zero valent zinc, inactive. Aluminum ion, Al3+, may substitute for (e.g. added as aluminum sulfate) for ferrous iron. It will be understood that iron is illustrative of a reactive solid. The present inventor believes that iron is particularly practical. However, the present inventor contemplates other treatment materials. For example, according to some embodiments, the treatment material is zinc. It will be understood that a reactive system may include the treatment material in zero valent form. According to some embodiments, the reactive system further includes a passivation reversal agent suitable for the zero valent form as may be advantageous.

According to some embodiments, a reactor includes an internal settling zone in communication with a reactive zone. The reactor is illustrated in schematic in FIG. 1. According to some embodiments, the internal settling zone uses gravitational forces to separate solids from liquids. According to some embodiments, mostly liquids remain in the settling zone. According to some embodiments, the internal settling zone is towards the top of the reactor (FIG. 1). According to some embodiments, communication with the reactive zone is via an inlet at the bottom of the internal settling zone. According to some embodiments, effluent is removed from the top region of the internal settling zone. According to some embodiments, the effluent is very clear. Magnetite is known to be black. Settling observed in an experiment over time is illustrated in FIG. 3 of the document “pilot test scale plan” appended hereto, which shows clearer separation of black material and clear fluid over time. The present inventor believes that settling for a separating method is particularly efficient. However, other suitable separating methods are contemplated.

According to some embodiments, a reactive zone includes a central conduit. The central conduit improves mixing. For example, according to some embodiments, the central conduit promotes convective motion.

Thus, according to some embodiments, the reactor system operates as a fluidized bed that employs a motorized stirrer in conjunction with a central flow conduit to create a circular flow within the reactor and provide an adequate mixing between reactive solids and wastewater. An internal settling zone was created to allow solid-liquid separation and return of the solid into the fluidized zone.

FIG. 1 is a schematic illustrating an embodiment of the system and process. A single-stage fluidized-bed system includes a fluidized reactive zone, an internal solid/liquid separating zone, an aerating basin, a final settling basin, and an optional sand filtration bed.

Still referring to FIG. 1, the fluidized zone is the main reactive space where reactive solid, in the form of particles, is completely mixed with wastewater and secondary reagent and where various physical-chemical processes responsible for toxic metal removal occur.

Still referring to FIG. 1, the internal settling zone is to allow particles to separate from water and be retained in the fluidized zone. For high density particles, an internal settling zone with a short hydraulic retention time is sufficient for complete solid/liquid separation. This eliminates the need of a large external clarifier and a sludge recycling system.

Still referring to FIG. 1, the aeration basin has two purposes: (1) to eliminate residual secondary reagent in the effluent from fluidized zone; and (2) to increase dissolved oxygen level. For a single-stage reactor, effluent from fluidized reactive zone will always contain certain amount of secondary reagent. Oxidation of secondary reagent will consume alkalinity and therefore will lower the pH. To accelerate oxidation of secondary reagent, the aeration basin should maintain a pH of above 7.0. Chemicals such as Ca(OH)2, NaOH and Na2CO3 could be used for pH control.

Still referring to FIG. 1, the final settling tank is to remove flocculent formed in the aeration basin. The floc (fluffy) settled to the bottom can be returned to the fluidized zone and transformed by secondary reagent into dense particulate matter.

Still referring to FIG. 1, upon final settling, a sand filtration bed may be used to further polish the treated water before discharge.

Still referring to FIG. 1, the post-FBR (fluidized bed reactor) stages (aeration-settling-filtration) may not be needed under certain operation conditions.

Referring now to FIG. 2, several fluidized-bed reactors can be combined to form a multi-stage treatment system. It is recommended that each stage maintain its own reactive solid. That is, the solids are separated in each stage. In order to achieve a separate solid system, each stage may have its own internal solid-liquid separation structure.

Still referring to FIG. 2, depending on operating conditions in the FBRs, the wastewater characteristics, and discharge standards, the post FBR treatments (aeration+final clarifier+sand filtration) may not be needed.

Although a multi-stage system is more complex and may result in a higher initial construction cost, a multi-stage fluidized-bed reactor system has several major advantages.

A multi-stage system can achieve higher removal efficiency than a single-stage system under comparable conditions. Further, the FGD wastewater may contain certain chemicals (i.e., phosphate) that may be detrimental to the high reactivity of the reactive solids. A multi-stage system can intercept and transform these harmful chemicals in the first stage and thus reducing the exposure of the subsequent stages to the negative impact of these chemicals. As such, a multi-stage configuration is more stable and robust.

A multi-stage configuration facilitates the control of nitrate reduction, for example in an iron-based system. In a single stage system, because the presence of dissolved oxygen carried in raw wastewater, it tends to be difficult to operate the system in a rigorous anaerobic environment. In a multi-stage system, stage 1 can remove virtually all dissolved oxygen; as a result, the subsequent stages can be operated under rigorous anaerobic environment.

A multi-stage system allows flexible control of different chemical conditions in each individual reacting basin. The chemical conditions in each reactive basin can be controlled by adjusting the pumping rate of supplemental chemicals and turning aeration on or off. A multi-stage system can be operated in a mode of multiple feeding points. Each stage may be operated under different pH and dissolved oxygen condition.

A multi-stage system will lower chemical consumption. In a single-stage complete-mixed system, secondary reagent in the reactor are desirably maintained at a relatively high concentration in order to maintain high reactivity of reactive solids. As a result, the residual secondary reagent in the effluent will be high. This means that more secondary reagent will be wasted and more NaOH (or lime) consumption will be required just to neutralize and precipitate the residual secondary reagent in the effluent. As a result, more solid sludge will be produced and waste disposal cost will increase. In a multi-stage system, residual secondary reagent from stage 1 can still be used in stage 2. In this case, secondary reagent can be added in a way that conforms to its actual consumption rate in each stage. As a result, it is possible to control residual secondary reagent in the effluent in the final stage to be much lower than the one in a single stage system.

Referring to FIG. 3, according to some embodiments, in the system and process illustrated by FIG. 1, the reactive solid includes zero valent iron (ZVI) and iron oxide mineral (FeOx), and the secondary reagent is Fe2+. Thus, referring to FIG. 3, a single-stage fluidized-bed ZVI/FeOx/Fe(II) system includes a fluidized reactive zone, an internal solid/liquid separating zone, an aerating basin, a final settling basin, and an optional sand filtration bed.

Still referring to FIG. 3, the fluidized zone is the main reactive space where ZVI and FeOx reactive solids are completely mixed with wastewater and dissolved Fe2+ and where various physical-chemical processes responsible for toxic metal removal occur.

Still referring to FIG. 3, the internal settling zone is to allow ZVI and FeOx to separate from water and be retained in the fluidized zone. Because of high density of fully or partially crystallized FeOx particles, an internal settling zone with a short hydraulic retention time would be suffice for complete solid/liquid separation. This eliminates the need of a large external clarifier and a sludge recycling system.

Still referring to FIG. 3, the aeration basin has two purposes: (1) to eliminate residual dissolved Fe2+ in the effluent from fluidized zone; and (2) to increase dissolved oxygen level. For a single-stage reactor, effluent from fluidized reactive zone will always contain certain amount of dissolved Fe2+. Oxidation of Fe2+ will consume alkalinity and therefore will lower the pH. To accelerate oxidation of dissolved Fe2+, the aeration basin should maintain a pH of above 7.0. Chemicals such as Ca(OH)2, NaOH and Na2CO3 could be used for pH control.

Still referring to FIG. 3, the final settling tank is to remove iron oxide flocculent formed in the aeration basin. The ferric oxide floc (fluffy) settled to the bottom can be returned to the fluidized zone and transformed by Fe2+ into dense particulate matter.



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stats Patent Info
Application #
US 20120273431 A1
Publish Date
11/01/2012
Document #
13509963
File Date
09/20/2010
USPTO Class
210719
Other USPTO Classes
252178, 210209, 210207, 210201, 210192
International Class
/
Drawings
8


Iron Oxide


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