Patent Application
20120241381
Federal and State regulations have established increasingly stringent standards for selenium levels in surface water discharges from mining and industrial operations. As reported in the “Review of Available Technologies for the Removal of Selenium from Water” (final report June 2010) to the North American Metals Council, regulations are limiting selenium to levels on the order of 1-5 μg/L in industrial water discharges—levels that are below established safe maximums for potable water. While there are a significant number of proven physical, chemical and biological treatment technologies to remove selenium from water, very few technologies have successfully and/or consistently demonstrated the ability to reduce selenium in water to less than 5 μg/L at any scale, much less in full-scale operation.
Waste rock in the overburden excavated in the mining process is a primary source of selenium in mine drainage water. The selenium is typically present in inorganic forms and leach into run-off water. Steps can be taken to lower selenium levels at the source by reducing contact and dwell time between the waste rock and water, but it is unlikely that any such precautionary measures can effectively limit selenium levels to less than 5 μg/L. Consequently, pre-discharge treatment of the waste water is needed before it can be released into the local watershed.
The present invention provides methods for the removal of selenium from water, and systems such as a water treatment facility for carrying out those methods.
The methods involve staged treatment of the contaminated water, in which a first selenium removal stage of iron co-precipitation technology is used to remove the bulk of selenium, followed by a polishing stage using a low-concentration selenium removal technology to achieve the final reduction to compliance level. In the preferred embodiments; the polishing stage can be hydride generation gas removal or ion-exchange resins in a metal absorption polymer media, or a combination of hydride generation and ion-exchange resin absorption media.
The systems for carrying out the methods include water treatment facilities with sequential tanks and controls for the selectable applications of the treatment steps within the flow of water through a sequence of the tanks.
The invention is potentially applicable to any discharge stream of water containing selenium, and is particularly suited for the mining industry.
Treatment Methods
The inventive method uses at least a dual-stage selenium reduction process. The first stage of the method is a bulk reduction process using a the known process of ferrihydrite adsorption, commonly referred to as “Iron Co-precipitation” or “Ferrihydrite Co-precipitation”. Ferrihydrite adsorption is a two step physical adsorption process in which a ferric salt is added to the water at proper pH and temperature conditions to form a ferric hydroxide and a ferrihydrite precipitate. A concurrent adsorption of selenium occurs on the surface of the precipitate and thus allows selenium to be removed from the water along with the precipitate; hence the name iron co-precipitation. The term “iron co-precipitation” will be used herein to indicate the process of ferrihydrite adsorption of selenium followed by a removal of the precipitate.
While iron co-precipitation is a low-cost and proven technology, it will not alone achieve the stringent reductions required under present and proposed regulations. The primary limitation on the co-precipitation process is the nature of the selenium in waste rock and, water run-off. Selenium typically occurs in one of four oxidation states: Se(0), Se(−II), Se(VI) and Se(IV). In buried and un-weathered mineral formations, it is most common as elemental selenium Se(0) and selenides such as HSc, which are selenium in the −2 oxidation state, Se(−II). In exposed and weathered waste rock, however, the selenium oxidizes primarily to two oxyanions; selenate (SeO4)−2 (which is selenium in the +6 oxidation state, Se(VI)) and selenite (SeO3)−2 (which is selenium in the +4 oxidation state, Se(IV)). Se(0) and Se(−II) are insoluble in aqueous solutions and therefore are more likely to be released as fine particulates to the atmosphere, but can be found as colloidal suspensions in run-off surface water. In a relatively neutral pH6 to 8 range, the primary selenium burden in the water column from, for instance, a mine discharge, will be dissolved selenite and selenate, with the possibility of suspended particles of elemental selenium.
The dilemma of iron co-precipitation as a single solution to selenium reduction is that the ferrihydrite precipitate aggressively adsorbs dissolved selenite and suspended selenium particles, but not selenate. This results in a diminishing returns scenario wherein adding more and more ferric salt fails to produce proportional reduction in the selenium level. As reported in a 2001 article, EPA/600/R-01/077 titled “SELENIUM TREATMENT/REMOVAL ALTERNATIVES DEMONSTRATION PROJECT: Mime Waste Technology. Program Activity III, Project 20”, ferrihydrite adsorption was tested by quantity of iron used, in three ranges described as low-iron, medium-iron and high-iron. Significant reduction was noted between low and medium iron (69 μg/L to 42 μg/L), but a much diminished change was observed between medium and high iron (42 μg/L to 35 μg/L). Although the iron co-precipitation was proven effective in reducing selenium to below the 50 μg/L. standard for potable water, it could not achieve levels below 5 μg/L consistently and without consuming excessive quantities of iron.
As a first stage treatment, however, the iron co-precipitation mechanism is a low-cost; effective and predictable process for bulk reduction of selenium. The selenium-bearing precipitate can be separated from the water stream by specific gravity, such as a weir trap, or other such gravity sedimentation filtration, prior to a second stage treatment to reduce the residual selenium concentration to levels below 5 μg/L.
Although ferric chloride is the preferred ferric salt, other compounds such as ferric sulfate can be substituted. The optimum pH range for iron co-precipitation treatment is pH 4 to 6, which is quickly reached when adding ferric chloride. To increase the iron loading, the pH can be maintained in the preferred range by concurrent addition of a non-interfering basic buffer material, such as sodium hydroxide, along with the ferric chloride. The plant systems described hereafter have mixing tanks for ferric salt mixing, and in one embodiment have a subsequent tank for sodium hydroxide/ferric salt mixing if needed to increase iron loading.
Iron co-precipitation is well known and has been designated by the EPA as a Best Available Demonstrated. Technology for selenium removal. However, it will often be incapable of reaching the new lower selenium limits if the water contains a significant fraction of selenate. In such instances, the selenate can be oxidized to selenite by co-mixing with an oxidizing agent such as potassium permanganate. One embodiment of a treatment plant described herein includes a tank for mixing permanganate into the water as the iron co-precipitation process is developing.
Following the addition of the iron salt and sufficient mixing and residence time to allow the breakdown to ferric hydroxide and ferrihydrite, the water is buffered to a pH of 8-9 to make insoluble the ferrihydrite precipitates. A polymer flocculent is then mixed into the water to link the precipitates into larger aggregates that can be separated from the water by gravity or filtration. In the preferred embodiment treatment plants, a floating flocking agent is used to form precipitate agglomerates that are less dense than the water and float to the surface of the water column. This allows the water to flow over a weir chute onto a roller filter, on which the precipitate is filtered out and conveyed to a sludge pan while the water passes trough the filter mesh into a collection tank for further treatment.
The selenium level will be greatly reduced following the stage 1 treatment, and levels below 50 μg/L are routinely achievable, but it may not be reduced to the 1-5 μg/L required by some current regulations, or at least not on a consistent basis over time. In order to consistently reach the reduced levels, the invention uses a second stage polishing process.
One of the presently preferred technologies for second stage processing technology is hydride generation. Hydride generation technology has previously been used in the analytic analysis of trace selenium concentrations. The measurement process, called “hydride generation atomic adsorption spectrometry” (HGAAS), determines trace selenium by generation of its gaseous hydride, hydrogen selenide (H2Se), using either metallic zinc or sodium borohydride as a reductant. The gaseous hydride is carried out of the liquid by an argon and entrained-air bubbler nozzle and into a hydrogen flame where the atomic fluorescence lines of selenium are detected by an atomic adsorption spectrometer. The attainable detection limits for selenium are 0.3 ng (15 pg/ml) with the zinc reductant and 0.4 ng (20 pg/ml) using the sodium borohydride.
While hydride generation technology is effective to extract a sample to measure for total selenium, it would not be practical as a bulk removal technology for selenium from waste water. As demonstrated herein, however, it is effective as a second stage removal technology where the bulk of selenium has previously been separated from the stream by the first stage iron co-precipitation process.
In one process and plant layout described herein, the water from the first stage collection tank is pumped into a stage 2 holding tank where the pH is adjusted to around 2.0 by adding nitric acid. The high acidity facilitates reaction with borohydride to release hydride gas, and reduces selenate to selenite. The water is then pumped from the holding tank to a bubbling tank through static mixing tubes. Sodium borohydride is injected into the mixing tubes and mixed into the water by the swirling action of the vanes in the tubes. The bubbling tank has air sparger nozzles at the bottom to force compressed air into the water column as a carrier gas to form bubbles that carry the hydride gas to the surface, where it is released.
The hydride generation can be conducted as a continuous process carrying the final traces of selenium off as hydride gas that can be burned for disposal or energy. In one plant embodiment the hydride gas is used in a hydrogen fuel cell to produce electrical power sufficient to run many of the automated functions. The selenium produced at the anode of the fuel cell by the deprotonation of the hydride gas can be captured and refined into an essential nutrient supplement for poultry and other animal feed.
. An alternative polishing stage is to use ion-exchange resins in a metal absorption polymer media as the second stage selenium removal. Iron exchange resin is a media which promotes electrostatic attraction between soluble ions and oppositely charged resin surfaces. In selenium adsorption, the anions selenate and selenite are collected at cationic charged sites in the resin media. A preferred example of such media is the open cell sponge media described in U.S. Pat. Nos. 5,096,946 and 5,002,984, a variation of which is currently sold by Cleanway Environmental Partners under the trade name MetalZorb.
In the water treatment plant systems described herein, the water collected after stage 1 treatment is pumped through an elongated tube or tubes containing a mesh bag filled with the iron exchange sponge material. The open celled sponge provides low impedance to water flow while bringing the water into contact with the resins that absorb both selenate and selenite. Since the bulk of selenium has been removed by stage 1 processing, the sponge material can be used continuously for an extensive time interval before needing replacement.
Description of Plant Layouts.
Pilot Plant:
A particular feature of the pilot plant layout, however, is that a modular treatment system can be contained in a transportable unit, and several mobile units can be connected in parallel at a particular pond. This gives the operator flexibility to increase or reduce discharge capacity.
In the upper left corner of
When the desired pH is detected in tank 1, a PLC controller 12 opens a valve to direct flow from tank 1 to the next mixing tank 2, and meters into tank 2 a ferric salt, preferably ferric chloride (FeCl3), to begin the formation of ferric hydroxide and the ferrihydrite precipitate which adsorbs on its surface dissolved selenite and any suspended selenium particles. The waste water then proceeds from tank 2 to the next mixing tank 3, where a polymer flocking agent is added and mixed throughout the tank to aggregate the precipitates into a floating sludge on the surface of the tank 3.
The outflow from the tank 3 pours onto a sludge belt filter 14 over a water collection trough 16. The belt filter is a wide mesh wire conveyor belt covered with a fine mesh filter cloth that traps the floating precipitate on the belt while allowing the water to pour through into the collection trough. The belt filter conveys the aggregate sludge into a sludge container 18. Water flows out of the collection trough and into tank 4, which is the beginning of the stage 2 treatment process if such additional treatment is required to reduce the residual selenium concentration to the very low 1-5 μg/L range required by some regulations. In this plant layout, the stage 2 processing uses hydride generation technology.
The water in tank 4, at this point, has undergone a bulk selenium removal in stage 1. To begin stage 2, the pH of the water solution is adjusted to an acidic pH within a range of pH 2 to 5, preferable about pH 2, by a chemical feed pump and mixer assembly 20 that controlled by a pH controller (not shown) associated the system PLC 12, that meters in nitric acid. When the pH of the water solution is stabilized at the desired pH, the PLC 12 opens a valve and pumps the adjusted water from tank 4 into a bubbling tank 5. Air injection nozzles 22 in the bottom of tank 5 inject a 12% sodium borohydride solution and pressurized air into the water, causing air bubbles to percolate to the surface. The sodium borohydride reacts with the selenium (and any other reactive metals such as mercury, antimony and arsenic that may be in trace amounts in the water) to form hydride gasses (e.g., hydrogen selenide), which are carried to the surface in the air bubbles. A vacuum hood 24 over the bubbling tank 5 captures the air/hydride stream and carries it out of the water system.
The processed water is then pumped from the bubbling tank 5 into a neutralization mixer tank 6, where another pH adjustment feed pump and controller 26 mix in sufficient buffer to create essentially neutral or slightly basic pure water, which is in turn pumped, into exit tank 7, from which water samples can be taken for compliance testing before the water is be released into a local groundwater drainage or natural stream.
The hydride gas air stream collected by the hood 24 is highly flammable and can be simply burned off into the atmosphere. In a preferred embodiment, however, the gas/air stream is used in a hydrogen fuel cell array (not shown) to produce a low voltage DC current sufficient to power the controls and pumps within the water treatment system. Thus, in a preferred embodiment, the mobile plant further includes a hydrogen fuel cell array fueled, at least in part, by the hydride gas. This embodiment makes the system largely self-contained once it is up and running. Line AC or auxiliary battery power may be needed at start up, and for heavier power demands outside of the system, but the power generated from a fuel cell array should be sufficient for the internal controls and pumps.
The selenium of the hydride gas will be released by a catalyst at the anode of the fuel cells, and only the hydrogen ions will pass through the electrolyte to the cathode. This selenium residue at the anode side is highly concentrated and optionally can be collected and refined into essentially pure elemental selenium that can, for instance, be sold for animal feed supplement.
Waste water from an acid mine drainage lagoon is pumped from the lagoon through a pre-treatment filter to remove suspended solid particles that might clog the treatment system 100. The filtered water is directed to tank 1A where it is mixed by a metering and mixing assembly 102 with a ferric salt, preferably ferric chloride, sufficient to lower the pH to an effective range for the breakdown to ferric hydroxide and ferrihydrite, preferably between pH 4.7 to 5.2. In the initial calibration of the plant, the water is allowed to proceed from tank 1A through tank 1B and tank 2 into tank 3, where the flocking agent is added, and then through the roller filter and on to the end of the system at tank 9 where it can be sampled for residual selenium level This initial calibration will give a baseline indication of how much selenium reduction can be achieved through simple iron co-precipitation, as compared to co-precipitation with high iron loading and/or permanganate oxidation. It is unlikely, however, that this baseline, reduction will be sufficient to consistently reach levels below 5 parts per billion. To achieve consistent discharge at these low levels, the process may need to be adjusted for more aggressive co-precipitation and stage 2 polishing.
The first option for increasing the removal through co-precipitation is increase iron loading by adding sodium hydroxide and ferric chloride in tank 1A, as the sodium hydroxide will allow more ferric chloride mixing while staying within the pH 4.7 to 5.2 range. This increased iron loading step should be tried as a first modification to determine what effect it causes in the selenium sampling from tank 9. If the change in selenium reduction is significant, then concurrent use of sodium hydroxide and ferric chloride in tank 1A is worth including in the process, as the additional iron loading yields commensurate reduction in selenium.
The next option to increase co-precipitation is adding an oxidizing agent in tank 1B. The preferred oxidizing agent is potassium permanganate. The purpose, of the permanganate is to convert selenate to selenite before precipitation.
The main formation of ferrihydrite precipitate takes place in tank 2 with the addition and mixing of sodium hydroxide to increase pH to 8 or above. The water from tank 2 flows into tank 3, where the polymer flocking agent is added and mixed through the water column. Tank 4 is the flock development tank, which provides sufficient residence time for the flocking agent to bind the ferrihydrite precipitate into aggregates that float to the surface of the tank.
As in the pilot plant, the surface sludge is separated from the water by pouring onto a roller filter 114 over a collection trough 116. The sludge is conveyed to a sludge container 118. The sludge is a polymer mix with high iron content, and can be de-watered for use in various industrial processes such as smelting
The water in the collection trough 116 can be sent directly to tank 8 for pH neutralization and to tank 9 for sampling to determine the residual level of selenium after the most aggressive iron co-precipitation. If the residual selenium is not consistently below the required level, it will be necessary to use a stage 2 polishing process.
In the layout of
Tank 7 contains the air sparging nozzles from which pressurized air is forced into the bottom of the tank and creates bubbles. The bubbles entrain the selenium hydride and other gases produced by the sodium borohydride, and carry the trapped gases to the surface, where the bubbles burst and release the gases into a vacuum hood 124.
The stage 2 treated water is then sent to tanks 8 and 9 for neutralization, sampling and discharge, as describe before.
In this alternative plant layout, the water from the stage 1 collection trough 116 is directed to a tank 130 that discharges through one or more elongated vessels 132 that have a hinged opening to insert a mesh bag 134 containing the MetalZorb sponges. The sponge media contains the ion-exchange resins. The open celled sponge provides low impedance to water flow while bringing the water into contact with the resins that absorb both selenate and selenite. Since the bulk of selenium has been removed by stage 1 processing, the sponge material can be used continuously for an extensive time interval before needing replacement. The discharge of the pipe(s) 132 is into the tanks 8 and 9 for neutralization, sampling and discharge, as describe before.
Although not expressly depicted, it should be easily apparent that a treatment plant layout could include both hydride generation and ion-exchange resin systems for stage 2 polishing. The two polishing systems could be run in series, in which the hydride generation discharge passes through the ion-exchange media, or one can be used as a back-up for the other to ensure compliance while one of the systems is shut down for maintenance.
The methods and plant systems described above contain some examples of the invention. The full scope of the invention is described by the claims which follow.
This application claims the priority of U.S. Provisional Application No. 61/454,772, filed on Mar. 21, 2011.
| Number | Date | Country | |
|---|---|---|---|
| 61454772 | Mar 2011 | US |
