WO2020043813A1

WO2020043813A1 - Process for removing selenium from waste water streams

Info

Publication number: WO2020043813A1
Application number: PCT/EP2019/073064
Authority: WO
Inventors: Tarakranjan Gupta; Mehak SINGLA; Manigandan PARTHASARATHI; Albert Joseph Hendrik Janssen; Wilhelmus Petrus Mul
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2018-08-31
Filing date: 2019-08-29
Publication date: 2020-03-05

Abstract

A process for removing selenium from a waste water stream, where the waste water stream comprises water, a selenite concentration of selenite anions (SeO₃ ^2-), and a selenate concentration of selenate anions (SeO₄ ^2-). The process comprises mixing an aqueous ferric solution with the waste water stream to form a first precipitate within a first reaction product that is separated to yield a first water product stream, having a first reduced selenite concentration, and a first sludge product. The first water product stream is then mixed with an aqueous ferrous solution to form a second precipitate within a second reaction product that undergoes a separation to yield a second water product stream, having a total selenium concentration of less than 10 ppbw, and a second sludge product.

Description

Process For Removing Selenium From Waste Water Streams

FIELD OF INVENTION OF THE INVENTION

[0001] This invention relates to a process for removing selenium from waste water streams, and more particularly, to a two-step iron addition process for removing selenium from refinery waste water streams.

BACKGROUND OF THE INVENTION

[0002] Selenium is a naturally occurring group 16 (VIA) element and is released into the environment by various sources such as the coal burning, oil and gas, irrigated agriculture, mining, and metallurgical industries. Pertaining to the oil and gas industry, crude oil containing seleniferous marine shales is rich in selenium, which is a major source of selenium in refinery effluent. During the various crude oil refining steps, selenium and other pollutants (e.g., oil residues, benzene, sulfur, arsenic, chromium, etc.) can be transferred from oil mixtures to waste water streams, where refinery units such as the desalter, coker, hydrotreater, and sour water stripper, provide a major source for selenium-contaminated waste water. For example, during crude oil processing, sour water that contains selenium compounds is stripped from an oil mixture and flows into a waste water treatment system. During treatment, the selenium compounds within the stripped sour water are oxidized to selenium oxyanions, i.e., selenate (Se0₄ ² ) and selenite (SeCF^2-), which are bioavailable with a high potential for bioaccumulation and toxicity. If harmful amounts of selenium remain even after treatment, selenium-contaminated treated waste water may be discharged into natural water bodies, thus, resulting in selenium contamination and adverse effects on aquatic and aquatic dependent wild life.

[0003] Federal and state mandates dictate the removal of selenium compounds from waste water streams before being discharged into natural water bodies. Over the years, three main methods have emerged for removing selenium from waste water including physical, biological, and chemical treatments. Physical treatment includes processes like ion-exchange, reverse osmosis, and membrane filtration. Biological treatment deals with organic carbon sources (e.g. aerobic and an-aerobic bacteria, algae, fungus) for conversion of soluble species (e.g., selenate and selenite) to insoluble species (e.g., elemental selenium). Chemical treatment includes adsorption, redox transformation, electrocoagulation, and precipitation of soluble and insoluble selenium species.

[0004] U.S. Patent 9,963,360 discloses processes for treating an aqueous solution to remove dissolved selenium oxyanions using an ion exchange process, a selenate precipitation process, or a combination of both the ion exchange process and the selenate precipitation process. For the selenate precipitation process, a selenate-containing aqueous solution is contacted with a ferrous iron or a ferrous/ferric iron mixture to co-precipitate or adsorb at least a portion of the selenate with the iron to thereby produce a selenium-iron precipitate and a selenium depleted discharge solution.

[0005] U.S. Patent No. 5,993,667 discloses a process of removing selenium from refinery water that includes mixing an aqueous solution of ferric sulfate or other soluble ferric salt with the refinery water to produce a first stream containing a precipitate consisting of ferric hydroxide and ferric oxyhydroxide. The first stream is further mixed with an aqueous permanganate solution, causing the oxidation of the selenium to selenite and the formation of a manganese dioxide precipitate. The selenite is adsorbed on both the manganese dioxide and the ferric hydroxide to form a second stream including selenium-containing solids.

[0006] U.S. Patent Application Publication No. 2012/0241381 discloses a dual-stage selenium reduction process. A first stage treatment uses an iron co -precipitation process to remove a bulk concentration of selenium from selenium contaminated water. In particular, ferric salt is added to form a ferric hydroxide and a ferrihydrite precipitate, which adsorbs dissolved selenite and suspended selenium particles, but not selenate. An oxidizing agent, i.e. potassium permanganate, is further introduced into the selenium contaminated water to convert the selenite to selenate. In a second stage treatment, the water from the first stage is treated by either a hydride generation process or an ion-exchange media, or a combination thereof, to achieve a lowered selenium concentration.

[0007] U.S. Patent No. 6,235,204 discloses chemical reduction and co -precipitation processes for removing oxy-anions of selenium from purge water where ferrous ions are added to the purge water to remove the oxy-anions of selenium.

[0008] These prior art processes typically provide post-treated waters where the concentration of selenium compounds remaining after treatment varies from 100-200 parts per billion (“ppbw”). None of the previously described approaches individually removes both the selenate and selenite concentrations in waste water streams to below 10 ppbw. For example, one of the previously described conventional techniques adds ferric chloride (FeCb) solution to feed streams to produce ferric oxide precipitate. Yet, this technique is only partially successful as it is more selective towards the removal of the selenite concentration rather than to the selenate concentration, especially, when the waste streams includes sulfates, nitrates, and/or other competing ions.

Accordingly, there is a continuing need for a selenium removal process that provides for the removal of both selenate and selenite concentrations from waste water to yield treated water having a selenium concentration of less than 10 ppbw before discharging the treated water into the environment.

SUMMARY OF THE INVENTION

[0009] Therefore, provided is a two-step iron addition process for removing selenium from refinery waste water streams. This process comprises removing selenium from a waste water stream that comprises water, a selenite concentration of selenite anions (SeCb² ), and a selenate concentration of selenate anions (Se0₄ ² ). The process includes mixing within a first mixing zone, an aqueous ferric solution, comprising water and ferric chloride, with the waste water stream and forming a first precipitate within a first reaction product, comprising water and the first precipitate. The first reaction product is separated within a first separating zone to yield a first water product stream, having a first reduced selenite concentration, and a first sludge product. The first water product stream is mixed within a second mixing zone with an aqueous ferrous solution, comprising water and ferrous chloride, to form a second precipitate within a second reaction product, comprising water and the second precipitate. The second reaction product is then separated within a second separating zone to yield a second water product stream having a total selenium concentration of less than 10 ppbw and a second sludge product.

DESCRIPTION OF THE DRAWINGS

[0010] Certain exemplary embodiments are described in the following detailed description and in reference to the drawings, in which: [0011] FIG. 1 is a schematic block flow diagram of an embodiment of the inventive two- step iron addition process removing selenium from a selenium-contaminated waste water stream containing an elevated selenium concentration;

[0012] FIG. 2 includes a bar chart presenting experimental data that demonstrates the effect of ferrous-to-selenate dosing ratios on selenium using the two-step iron addition process at pH 9;

[0013] FIG. 3 includes a bar chart presenting experimental data that demonstrates the effect of ferrous-to-selenate dosing ratios on selenium using the two-step iron addition process at pH 8.

DETAILED DESCRIPTION

[0014] Waste water streams produced during the refining of crude oil may be introduced into the environment after conventional treatment to remove contaminants provided that the amount of selenium present in the treated waste water streams is non-existent or so small that it is of no consequence. Yet, in areas where substantial concentrations of selenium are naturally present during the refining of crude oils, waste water streams produced from these areas may present a problem when selenium concentrations exceed environmental standards. As a result, selenium-removal treatments should be performed to reduce the selenium concentration to acceptable standards prior to introducing the streams into natural water bodies.

[0015] The invention is a two-step iron addition process that provides for removing selenium compounds from waste water streams. These selenium compounds can include selenate and selenite anions and elemental selenium. The first iron addition step of the two- step process applies an aqueous ferric solution to reduce the selenite concentration of the waste water. Thereafter, the second iron addition step of the two-step process applies an aqueous ferrous solution to the recovered water from the first iron addition step to reduce its selenate concentration and to further reduce the selenite concentration. This inventive two-step process is potentially applicable to any discharge water stream containing selenium, including effluents, process waste waters, aqueous solutions, and the like, generated during oil and gas operations or other industry operations. As used herein, the term "selenium" refers to all forms of selenium, including selenium compounds, selenate and selenite anions, and elemental selenium. [0016] The two-step process exhibits enhanced removal of total selenium from waste water by removing a larger fraction of both the selenate and selenite concentrations from waste water than prior art processes are able to remove. The two-step process effectively reduces the total selenium concentration, including both the individual selenate and selenite

concentrations, to provide a treated water product having less than 10 ppbw selenium. A surprising feature of one embodiment of the inventive process is that, by controlling the pH of the iron addition and mixing steps and of the reaction products produced during the two-step process, the individual selenate and selenite concentrations are reduced to less than 10 ppbw and even less than 5 ppbw. Another unexpected benefit of the two-step process is that it provides a decrease in the volume of sludge that is produced by the process when compared to prior art processes. A further unexpected benefit of the two-step process is that it provides quicker settling properties associated with the produced sludge as opposed to typical processes that have slower sludge settling times.

[0017] In crude processing, often a waste water stream is stripped from the crude oil where non-volatile selenium compounds within the waste water stream are converted to

selenocyanate (SeCN ) and selenium hydride (SeH₂). These compounds may be present in the waste water stream at a concentration in the range of from 700-5000 ppbw. The SeCN and SeH₂ compounds of the waste water stream are then typically oxidized into selenate (Se0₄ ² ) and selenite (SeCb² ) compounds that require removal by a waste water treatment facility.

[0018] The inventive two-step process provides for the treatment of these waste water streams that comprise water, a selenate concentration of selenate anions, and a selenite concentration of selenite anions. The total selenium concentration in the waste water stream can be in the range of from 20 ppbw up to 1,000 ppbw. More typically, the total selenium concentration of the waste water stream is at least 50 ppbw and less than 800 ppbw. The selenite concentration in the waste water stream typically is at least 20 ppbw and less than 500 ppbw, but, more typically, the selenite concentration is in the range of from 50 to 400 parts per billion by weight (ppbw). The selenate concentration in the waste water stream typically is at least 20 ppbw and less than 500 ppbw, but, more typically, the selenate concentration is in the range of from 50 to 400 ppbw. The ratio of selenate to selenite varies between refineries and sometimes within different treatment facilities of the same refinery depending on the operations, processes, and types of treatments. The waste water stream may further include other pollutants and contaminants, such as sulfates, nitrates, bicarbonates, chlorides, and the like.

[0019] The sources of the waste water and selenium can be found in coker effluent, extracted groundwater, boiler blowdown, cooler blow down, storm water, wash water, process water wash streams, and any combination of these. Stripped sour water and desalter effluent are often the two most dominating waste water sources that contain elevated selenium levels. One or more of the source streams may originate from various production areas and can be collected from and contained within storage tanks, containment ponds, or any other type of collection/containment system. The type of containment system used may depend on the number and volume of contaminated streams generated and the degree of treatment desired.

[0020] In the first iron addition step of the process, a waste water stream from a first containment system is introduced along with an aqueous iron solution into a first mixing zone. The first mixing zone is defined by any suitable means for mixing the waste water stream with an aqueous iron solution to provide a first reaction product, comprising water and a first precipitate. Suitable mixing means are those capable of providing for liquid-liquid mixing of the aqueous iron solution with the waste water stream such as static mixers, in-line mixers, in tank mixers, and an agitated tanks, vessels and other containers. In the first iron addition step, an aqueous ferric solution stored, for example in a feed tank, is introduced into the first mixing zone along with the waste water stream wherein the two are mixed together to form the first reaction product, comprising water and a first precipitate. It is preferred for the first iron addition step to be conducted within a first mixing vessel or container or tank that is equipped with at least one agitator providing for mixing of the waste water and aqueous iron solution.

[0021] The mixing of the aqueous ferric solution with the waste water stream forms a first reaction product within the first mixing zone that reduces the selenite concentration of the waste water stream. The aqueous ferric solution includes dissolving soluble iron salt, ferric chloride (FeCb), in water. The ferric chloride solution may also be prepared by dissolving commercially available ferric chloride hexahydrate (FeCl₃-6H₂0) in the appropriate amount of water.

[0022] The aqueous ferric solution that is mixed with the waste water stream typically will have a molar concentration of ferric chloride in the range of from 0.5 millimoles per liter (mmol/L) to 50 mmol/L, preferably, from 1 to 30 mmol/L, and most preferably, from 1.5 to 20 mmol/L. The amount of aqueous ferric solution mixed with the waste water stream should be such as to provide a dosing ratio of ferric-to- selenite that is in the range of from about 1,000:1 to 5,000:1 moles of ferric-to-moles of selenite to effectively reduce the selenite concentration in the waste water steam. It is preferred to apply in the first mixing step a ferric-to-selenite dosing ratio in the range of from 1,000:1 to 4,000:1 moles of ferric-to-moles of selenite and, most preferred, from 1,000:1 to 3,500:1 moles of ferric-to-moles of selenite to reduce the selenite concentration in the waste water steam.

[0023] In the first iron addition or mixing step, the selenite concentration of the waste water stream is removed from the waste water stream by a precipitation reaction with the ferric chloride. This precipitation reaction occurs by the reaction of the ferric chloride of the aqueous ferric solution with the selenite of the waste water stream in the first mixing zone to generate a first precipitate that may comprise an iron precipitate that is a crystalline form of ferric hydroxide.

[0024] The volumetric flow rate of the waste water stream and aqueous ferric solution introduced into the first mixing zone should be such as to provide a sufficient residence time for the selenite of the waste water stream to react with the ferric chloride of the aqueous ferric solution to produce the first precipitate of the first reaction product. The residence time of the feed to the first mixing zone is, thus, at least 0.1 hour. More desirably, however, the residence time of the feed to the first mixing zone is in the range of from 0.5 to 50 hours, preferably between 1 to 20 hours.

[0025] Controlling or maintaining the pH of first reaction product in the mixing zone within a specific range is important to the formation of the first precipitate and removal of selenium from the waste water stream. Minimization of sludge yield and enhancement in sludge removal are additional unexpected benefits provided by controlling the pH of the first reaction product within the specific range. To achieve these benefits, the pH of the first reaction product within the mixing zone should be basic, but more typically, the pH is maintained within the range of from 7 to 11. Preferably, the pH is maintained within the range of from 7.5 to 10, and, more preferably, from 7.5 to 9.5.

[0026] To control the pH of the first reaction product, a first pH-regulating solution is introduced into the first mixing zone along with the waste water stream and aqueous ferric solution in amounts so as to maintain the pH within the required range. Examples of suitable pH-regulating solutions include sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂), or other similar basic solutions. The first pH-regulating solution may be stored in a feed tank or other conventional storage device and pumped into the first mixing zone.

[0027] The first precipitate is formed at any temperature within a broad range, since the formation of the first precipitate is not dependent on temperature. For instance, the process may be conducted at a temperature related to the surrounding environmental conditions such as from -5°C to 50°C, or in other words, at ambient temperatures. Typically, the ambient temperature is within the range of from 0°C to 30°C.

[0028] The precipitate formation in the first mixing step results in at least 50% of the selenite anions contained in the waste water stream to react with the introduced iron to form the first precipitate. This first stage more typically provides for the precipitation and removal of at least 85% of the selenite anions contained in the waste water stream. Preferably, at least 95% of the selenite is removed from the waste water stream by the precipitation reaction. Accordingly, the first precipitate of the first reaction product comprises the iron precipitate and the co-precipitated selenite anions which are physically adsorbed onto the surface of the iron precipitate.

[0029] While it is recognized that the mixing and separating steps of the first stage of the process may be performed by use of a single equipment item, it is preferred to individually perform these process steps with separate equipment. But, an embodiment of the process may include combined mixing and separating steps. This combination may include, for example, the use of single means providing for both mixing the waste water stream and aqueous ferric solution to yield a first reaction product and separating the first reaction product to yield a first water product stream and a first sludge product. This single means defines a single mixing and separation zone. An example of this embodiment includes introducing the waste water stream and aqueous ferric solution into a single zone that provides for both mixing the feeds and separating the resulting first reaction product to yield from the single zone a first water product stream and a first sludge product.

[0030] In a preferred embodiment of the process, the first reaction product flows from the first mixing zone into a first separating zone. The first separating zone is defined by any suitable means for separating the first reaction product to yield a first water product stream, having a first reduced selenite concentration, and a first sludge product. Suitable separating means are those capable of providing for solids-liquid separation of the first reaction product to provide the first product stream and the first sludge product. Examples of separating means include clarifiers such as sedimentation tanks of any suitable configuration providing for gravity separation of the first precipitate from the water component of the first product stream, filter separation systems, including filter presses, and centrifugal separation systems. It is preferred to conduct the first separation step of the process within the first separating zone defined by a clarifier.

[0031] The volumetric flow rate of the first reaction product through the first separating zone should be such as to provide a sufficient residence time for the first precipitate to settle for recovery as a first sludge product. This residence time should exceed 0.5 hours. The preferred residence time is between 1 to 50 hours, or more preferably between 2 to 10 hours.

[0032] The first sludge product comprises the first precipitate, including the iron precipitate and the adsorbed selenite anions, and any other contaminants that may be removed by conventional solid-liquid separation techniques. The first sludge product may be transported off-site for disposal or may require additional treatment before it is suitable for final disposal.

[0033] The separated first water product stream has a first reduced selenite concentration as compared to the selenite concentration of the waste water stream. The precipitation and removal of the selenite provided by the mixing and separating steps of the first stage of the process as described above provide for a first reduced selenite concentration that typically is less than 100 ppbw, depending upon the concentration of selenite in the waste water stream. More typically, the first reduced selenite concentration of the first water product stream is less than 50 ppbw, and, most typically, it is less than 25 ppbw. The lower limit for the first reduced selenite concentration is usually greater than 1 ppbw or 2 ppbw. The addition of the aqueous ferric solution in the first stage of the process removes little of the selenate concentration of the waste water. So, most of the total remaining selenium concentration is as a selenate concentration. The inventive process reduces not only the selenite concentration but also the selenate concentration to provide a total selenium concentration in the final treated waste water provided by the two-step process that is less than 10 ppbw.

[0034] To reduce the selenate concentration and to further reduce the remaining selenite concentration of the treated water of the process, a second iron addition step is carried out in which the first water product stream having the first reduced selenite concentration passes from the first separating zone and is introduced into a second mixing zone. The second mixing zone is defined by any suitable means for mixing the first water product with an aqueous ferrous solution. Suitable mixing means are those capable of providing for liquid-liquid mixing of the first water product with the aqueous ferrous solution such as static mixers, in line mixers, in-tank mixers, and agitated tanks, vessels and other containers.

[0035] In the second iron addition step, an aqueous ferrous solution stored, for example in a feed tank, is introduced into the second mixing zone along with the first water product wherein they are mixed together to form the second reaction product, comprising water and the second precipitate. It is preferred to conduct the second iron addition step within a second mixing vessel or container or tank that is equipped with at least one agitator providing for mixing of the first water product stream with the aqueous ferrous solution.

[0036] The aqueous ferrous chloride solution that is mixed with the first water product acts as an oxidizing/reducing (redox) agent to induce redox reactions. This added ferrous chloride is oxidized within the second mixing zone in the presence of the first water product stream to form ferrous-ferric hydroxide. The formed ferrous-ferric hydroxide acts as an active reducing agent to reduce selenate to selenite and insoluble elemental selenium. Mixing of the aqueous ferrous solution within the second mixing zone with the first water product stream from the first stage of the process reduces its selenate concentration and further reduces remaining selenite concentration to form a second reaction product, comprising a second precipitate and water.

[0037] The second precipitate comprises insoluble elemental selenium and ferrous-ferric hydroxide, both of which are later separated from the water component of the second reaction product. The formation of the second precipitate and its later separation provides for removal and reduction of at least 50% of the selenate anions contained in the first water product stream. The process, however, is capable of removing at least 85%, and most preferably, it provides for removing at least 95% of the selenate anions contained in the first water product stream.

[0038] The mixing of the aqueous ferrous solution with the first water product stream from the first iron addition step of the process forms a second reaction product within the second mixing zone that reduces the selenate concentration of the first water product stream. A commercially available aqueous ferrous chloride solution may be suitability used in the process. Also, the ferrous chloride solution may also be prepared by dissolving a commercial FeCl₂ salt, e.g., FeCl₂4H₂0, in the appropriate amount of water to provide an aqueous ferrous chloride solution.

[0039] The aqueous ferrous solution that is mixed with the first water product stream will have a molar concentration of ferrous chloride in the range of from 5 to about 50 mmol/L, but preferably, from 15 to 40 mmol/L. The amount of aqueous ferrous solution mixed with the first water product stream should be such as to provide a dosing ratio of ferrous-to-selenate ranging from about 5,000:1 to about 28,000:1 moles of ferrous-to-moles of selenate to effectively reduce the selenate concentration in the first water product stream. It is preferred to apply in the second mixing step a ferrous-to-selenate dosing ratio in the range of from

10,000:1 to 27,500:1 moles of ferrous-to-moles of selenate, and, most preferred, from

15,000:1 to 26,500:1 moles of ferrous-to-moles of selenate to reduce the selenate

concentration in the first water product stream.

[0040] The volumetric flow rate of the first water product stream and aqueous ferrous solution introduced into the second mixing zone should be such as to provide a sufficient residence time for the selenate of the first water product stream to react with the ferrous chloride of the aqueous ferrous solution to produce the second precipitate of the second reaction product. The residence time of the feed to the second mixing zone is, thus, at least 0.1 hour. More desirably, however, the residence time of the feed to the second mixing zone is in the range of from 0.5 to 50 hours, preferably between 1 and 20 hours.

[0041] Controlling or maintaining the pH of the second reaction product of the second mixing zone to within a specific range is important to the formation of the second precipitate and removal of selenium from the waste water stream. To achieve these benefits, the pH of the second reaction product within the second mixing zone should be basic, but more typically the pH is maintained within a range required to form the ferrous-ferric hydroxide and elemental selenium within the second reaction product. The pH of the second reaction product is thus maintained within the range of from 7 to 11. More preferably, the pH is controlled within the range of from 7.5 to 10, and most preferably from 7.5 to 9.5.

[0042] While it is recognized that the mixing and separating steps of the second stage of the process may be performed by use of a single equipment system, it is preferred to individually perform these process steps with separate equipment. But, an embodiment of the process may include combined mixing and separating steps. This combination may include, for example, the use of single means providing for both mixing the first water product stream and aqueous ferrous solution to yield a second reaction product and separating the second reaction product to yield a second water product stream and a second sludge product. This single means defines a single mixing and separation zone. An example of this embodiment includes introducing the first water product stream and aqueous ferrous solution into a single zone that provides for both mixing the feeds and separating the resulting second reaction product to yield from the single zone a second water product stream and a second sludge product.

[0043] In the preferred embodiment of the process, the second reaction product flows from the second mixing zone into a second separating zone. The second separating zone is defined by any suitable means for separating the first reaction product to yield a first water product stream, having a first reduced selenite concentration, and a second sludge product. Suitable separating means are those capable of providing for solids-liquid separation of the second reaction product to provide the second product stream and the second sludge product.

Examples of separating means include clarifiers such as sedimentation tanks of any suitable configuration providing for gravity separation of the second precipitate from the water component of the second product stream, filter separation systems, including filter presses, and centrifugal separation systems. It is preferred for the second separation step of the process to be conducted within the second separating zone defined by a clarifier.

[0044] The volumetric flow rate of the second reaction product through the second separating zone should be such as to provide sufficient residence time for the second precipitate to settle for recovery as a second sludge product. This residence time should exceed 0.5 hours. The preferred residence time is between 1 to 50 hours, or more preferably between 2 to 10 hours.

[0045] The second sludge product comprises the second precipitate, including the formed ferrous-ferric hydroxide, which acts as an active reducing agent to reduce selenate to selenite. The second sludge product may be transported off-site for disposal or may require additional treatment before it is suitable for final disposal.

[0046] The second water product stream can include a second reduced selenite

concentration that is less than 5 ppbw, and, even less than 2 ppbw or 1 ppbw. The reduced selenate concentration in the second water product stream is less than 10 ppbw, and, preferably, it is less than 5 ppbw. The second water product stream can include a total selenium concentration of less than 10 ppbw, preferably less than 5 ppbw for both the individual selenate and selenite concentrations. The second water product stream can be pumped into a second containment system where it is recycled for further use as a treated water stream in other operations or discharged into natural water bodies.

[0047] As discussed herein, one of the factors that determines the effectiveness of the process is appropriate control of the pH conditions of the first and second iron addition reaction steps to fully generate the precipitates and to induce the adsorption and reduction reactions. The adsorption of selenite anions onto the first precipitate within the first mixing zone occurs at neutral to slight alkaline pH values. The formation of the second precipitate, including ferrous-ferric hydroxide and reduced selenate that is predominantly insoluble elemental selenium, within the second mixing zone takes place under alkaline pH condition. The use of the aqueous ferric and the aqueous ferrous solutions in the reaction steps may increase the acidity of the related reaction products that flow from the first mixing zone and the second mixing zone. This increase in acidity can require application of the pH-regulating solutions in order to control the pH of the reaction mixtures.

[0048] It is surprising that, by maintaining the pH of the first and second reaction products to within a specific range in the operation of the inventive two-step process, the removal of selenite and selenate concentrations from a waste water stream is enhanced over the selenium removal capabilities of conventional selenium removal processes. Thus, it is desirable to control the pH of the second reaction product to within the important pH range. A second pH- regulating solution is used to control the pH by adding it to the second mixing zone. The pH of the second reaction product within the second mixing zone should be maintained within the range of from 7 to 11, preferably from 7.5 to 10, and most preferably from 7.5 to 9.5.

[0049] FIG. 1 is a schematic block flow diagram of an embodiment of the inventive selenium removal process 100 for removing selenium from a selenium-contaminated waste water stream containing an elevated selenium concentration, including both elevated selenate and selenite concentrations. The schematic of FIG. 1 depicts the flow of the waste water stream as it is processed in two separate and sequential iron addition steps whereby each product generated after each reaction is subjected to separation techniques. [0050] As shown in FIG. 1, the waste water stream flows via line 102 into first mixing zone 104, which is defined by first mixing vessel 106. The waste water stream can be pumped from a containment pond, storage tank, or any vessel known to those skilled in the art for containing the waste water. During the first iron addition step, an aqueous ferric solution, comprising water and ferric chloride (FeCb), flows via line 108 into first mixing zone 104 within which the aqueous ferric solution and waste water stream are mixed. First mixing vessel 106 provides means for mixing the waste water stream and the aqueous ferric solution preferably to provide a homogenous mixture within first mixing zone 104. The aqueous ferric solution reacts with the waste water stream within the first mixing zone 104 to generate a first reaction product comprising water and a first precipitate. The first precipitate may include ferrihydrite precipitation, which is a crystalline form of ferric hydroxide, and a co-precipitation of the selenite anions of the waste water stream adsorbed onto the surface of the ferrihydrite precipitate.

[0051] To fully generate the first precipitate, a first pH-regulating solution can be added via line 109 to first mixing zone 104 to maintain a basic pH value that is in the range of from 7 to 11, more preferably from 7.5 to 10.

[0052] The first reaction product exits first mixing zone 104 via line 110 and is introduced into first separating zone 112 defined by first separation device 114. The first separation device 114 provides means for separating the first reaction product to yield a first water product stream and a first sludge product. First separation device 114 can include any conventional device, including gravity sedimentation, mechanical pressing, or filtration devices, to separate the first reaction product.

[0053] The first sludge product is composed of the first precipitate and the adsorbed selenite anions and exits first separation device 114 via line 116 to be transported for off-site disposal or for additional treatment before final disposal. The first water product stream and passes from the first separation device 114 via line 118 and into second mixing vessel 120.

[0054] While the first water product stream includes a reduced selenite concentration, the addition of the aqueous ferric solution at the first iron addition step does not, however, reduce the selenate concentration to acceptable levels. During the second iron addition reaction step to reduce the selenate concentration, an aqueous ferrous solution, comprising water and ferrous chloride (FeCl₂), flows via line 122 into second mixing zone 124 defined by second mixing vessel 120. Second mixing vessel 120 provides a means for mixing the first water product stream and the aqueous ferrous solution to preferably provide a homogenous mixture. The aqueous ferrous solution reacts with the first water product stream within the second mixing zone 124 to form a second reaction product that includes a second precipitate.

[0055] The second reaction product is formed when the ferrous ions of the aqueous ferrous solution are oxidized to ferric ions to form ferrous-ferric hydroxide. The ferrous-ferric hydroxide acts as an active agent to reduce the selenate anions to selenite anions which are further reduced to insoluble elemental selenium. The second reaction product is comprised of a second precipitate and water.

[0056] In order to fully induce the adsorption and reduction reactions, a second pH- regulating solution can be added via line 126 to second mixing zone 124 of second mixing vessel 120 to maintain a basic pH value that is in the range of from 7 to 11, preferably from 7.5 to 10.5.

[0057] The second reaction product exits second mixing vessel 120 via line 128 and is introduced into second separating zone 130 defined by second separation device 132. The second separation device 132 provides means for separating the second reaction product to yield a second water product stream and a second sludge product. Second separation device 132 can include any conventional device, including gravity sedimentation, mechanical pressing, or filtration devices, to separate the first reaction product.

[0058] The second sludge product is composed of the second precipitate, which includes the ferrous-ferric hydroxide and the insoluble elemental selenium particles, and possibly other contaminants removed during the process. The second sludge product exits second separation device 132 via line 134 for transportation to off-site disposal or for additional treatment before final disposal.

[0059] The second water product stream passes from the second separation device 132 via line 136 to flow into a containment pond, discharged into natural water bodies, or recycled as treated water for use in additional operations. The second water product stream has a reduced selenium concentration of less than 10 ppbw, preferably, less than 5 ppbw, which may comply with environmental standards that place restrictions on the total selenium concentration within discharged water. The reduced selenium concentration can include both a reduced selenate concentration and a reduced selenite concentration of less than 10 ppbw, preferably, less than 5 ppbw after carrying out the first and second iron addition steps.

EXAMPLE 1

[0060] This Example 1 describes the experiments that were run, and which were based on pilot trials that illustrate certain features of the inventive two-step iron addition process for removing both selenite and selenate concentrations from a waste water stream. This process includes the addition of an aqueous ferric solution to a first mixing zone of the first addition step of the process followed by a second addition of an aqueous ferrous solution to a second mixing zone of the second addition step of the process.

[0061] The composition of the feed used in each of the experiments is shown in Table 1.

TABLE 1 - Composition of Waste Water Feed Used in the Experiments

[0062] The experiments were conducted in continuously stirred tank reactors (CSTR) that provided for homogeneous distribution of the precipitates that were formed in situ within the first and second mixing zones. The experiments were conducted at steady-state, continuous- flow conditions and at a temperature in the range of from 20 to 30°C. In all the experiments, the feed flow rate was maintained at 100 liters/hour (L/hr) providing a liquid residence time of at least 30 minutes within each mixing zone. Some of the experiments were run while maintaining a constant reaction product pH of 8 and other of the experiments were run while maintaining a constant reaction product pH of 9. Each experiment was carried out for a time- period of 5 to 6 hours (hr). [0063] The dosing rates of the aqueous ferric and ferrous solutions were each independently controlled to provide effective doses to remove both the selenite and selenate concentrations. In each experiment, the molar concentration of the aqueous ferric solution introduced into the first mixing vessel was 4.99 mmol total ferric per liter. The aqueous ferric solution was introduced at a rate of one liter per 100 liters of waste water feed to provide a ratio of 3,400:1 moles of ferric-to-moles of selenite.

[0064] The molar concentrations of the aqueous ferrous solutions introduced into the second mixing vessel, however, were different for each of the individual experiments. There were five different molar ferrous addition rates applied in the experiments. The aqueous ferrous solutions were introduced into the second mixing vessel at a rate of one liter per 100 liters of the first water product stream recovered from the ferric addition stage. The molar concentrations of the aqueous ferrous solutions were 7.60 mmol/L (5,000:1 moles of ferrous- to-moles of selenate), 15.2 mmol/L (10,000:1 moles of ferrous-to-moles of selenate), 22.8 mmol/L (15,900:1 moles of ferrous-to-moles of selenate), 30.4 mmol/L (21,200:1 moles of ferrous-to-moles of selenate), and 38.0 mmol/L (26,550:1 moles of ferrous-to-moles of selenate). The amount of aqueous ferrous solution mixed with the first water product stream provided a dosing ratio of ferrous-to- selenate in the range of from 5,000:1 to 28,000:1 moles of ferrous-to-moles of selenate to effectively reduce the selenate concentration in the first water product stream.

[0065] These experiments demonstrate that the inventive process overcomes many limitations of conventional selenium removal techniques by providing treated water having selenite and selenate concentrations of less than 10 ppbw. While the first mixing step of the experiments applies an aqueous ferric solution to reduce the selenite concentration of the feed, the test results presented in the following Examples focus on the selenium concentrations of both selenite and selenate in the treated water after carrying out the second mixing step of the process. The second mixing step of the process includes the addition of an aqueous ferrous solution to reduce the selenate concentration.

EXAMPLE 2

[0066] This Example 2 presents the results of the experimental testing described in Example 1. Table 2 presents data showing the total selenium concentration, including the selenite and selenate concentrations, in an untreated feed and in treated water after conducting the two-step iron addition process at a pH of 9.

[0067] After carrying out the first mixing step (“Step 1”) of the iron addition process in the first mixing zone, a first water product stream having a reduced selenite concentration was generated and introduced into the second mixing zone. The molar concentration of the aqueous ferric solution added was 4.99 mmol/L and an effective ferric-to- selenite dosing ratio to reduce the selenite concentration to less than 5 ppbw was about 3,400:1 moles of ferric-to- moles of selenate. The second mixing step (“Step 2”) included adding an aqueous ferrous solution to the second mixing zone containing 100 liters of the first water product stream at a pH of 9. In the several individual experiments, the molar concentrations of the aqueous ferrous solutions added were 7.60 mmol/L (5,000:1 moles of ferrous-to-moles of selenate), 15.2 mmol/L (10,000:1 moles of ferrous-to-moles of selenate), 22.8 mmol/L (15,900:1 moles of ferrous-to-moles of selenate), 30.4 mmol/L (21,200:1 moles of ferrous-to-moles of selenate), and 38.0 mmol/L (26,550:1 moles of ferrous-to-moles of selenate). The aqueous ferrous solutions were added to the second mixing zone in an amount of one liter per 100 liters of the first water product stream.

[0068] Table 2 presents the results of this experiment showing the removal of selenate at varying ferrous-to- selenate dosing ratios. As shown, effective removal of selenate to below 5 ppbw initially occurs at a ferrous-to- selenate dosing ratio of 15,900:1 moles of ferrous-to- moles of selenate. Accordingly, after administering effective ferric and ferrous doses, the inventive two-step iron addition process reduces the total selenium concentration in a feed having a high selenium concentration to a concentration of less than 5 ppbw in the treated water.

TABLE 2

[0069] FIG. 2 includes a bar chart presenting experimental data that demonstrates the effect of ferrous-to-selenate dosing ratios on selenium using the two-step iron addition process at a pH of 9 as described in this Example 2 and Table 2. FIG. 2 shows the extent of selenium removal based on the ferrous dosing rates at Step 2. The ferrous dosing and flow rates were different for each experiment and varied over a range that included 7.6 mmol/L (flow rate =

1.7 ml/min), 15.2 mmol / L (flow rate = 3.3 ml/min), 22.8 mmol / L (flow rate = 5.0 ml/min), 30.4 mmol/ L (flow rate = 6.7 ml/min), and 38.0 mmol/ L (flow rate = 8.4 ml/min). The ferric - to-selenite dosing ratio was the same for each experiment and was held at 3,400:1 moles of ferric-to-moles of selenite, respectively to reduce the selenite concentration to less than 5 ppbw.

[0070] As shown in FIG. 2 and described by the data of Table 2, the total selenium concentration was reduced to about 5 ppbw at ferrous-to-selenate dosing ratios of 15.900:1 moles of ferrous-to-moles of selenate, 21,200:1 moles of ferrous-to-moles of selenate, and 26,550:1 moles of ferrous-to-moles of selenate. At lower ferrous-to-selenate dosing ratio, including 5,000:1 moles of ferrous-to-moles of selenate and 10,000:1 moles of ferrous-to- moles of selenate, the total selenium concentration remaining after treatment was greater than 20 ppbw. EXAMPLE 3 (COMPARATIVE)

[0071] This Example 3 presents results from running a comparative experiment applying a single-step method for removing selenium from a waste water feed having a concentration of selenium. The single-step method is conducted by the co-addition of ferric chloride and ferrous chloride to the waste water feed.

[0072] Table 3 presents the results of this comparative experiment. Shown is the total selenium concentration, including the selenite and selenate concentrations, before and after conducting the single stage co-addition process at a pH of 9. The co-addition technique includes adding both ferric chloride and ferrous chloride in a single step to a single reactor. The ferric and ferrous concentrations, ferric-to- selenite dosing ratios, and ferrous-to-selenate dosing ratios were the same as those discussed in Example 2.

TABLE 3

[0073] The results presented in Table 3 show that the co-addition process fails to reduce the selenate concentration to below 10 ppbw at any of the ferrous-to-selenate dosing rates.

EXAMPLE 4

[0074] This Example 4 presents the results of the experimental testing for the same two- step iron addition process described in Example 2 except that the process steps were performed at a pH of 8. [0075] In two test runs, the molar concentrations of the aqueous ferrous solutions added to the first water product stream include 30.4 mmol/L (21,200:1 moles of ferrous-to-moles of selenate) and 38.0 mmol/L (26,550:1 moles of ferrous-to-moles of selenate) to effectively reduce the selenate concentration in the first water product stream. As shown in the Table 4, effective removal of selenate concentrations to below 5 ppbw occurs at a ferrous-to-selenate dosing ratio of 26,550:1 moles of ferrous-to-moles of selenate. Accordingly, after

administering effective ferric and ferrous doses, the inventive two-step iron addition process reduced the total selenium concentration in the waste water feed, having a high selenium concentration, to less than 5 ppbw.

TABLE 4

[0076] FIG. 3 includes a bar chart presenting experimental data that demonstrates the effect of ferrous-to-selenate dosing ratios on selenium using the two-step iron addition process at pH 8 as described in Example 2 and Table 4. To see the extent of selenium removal based on the ferrous dosing rates at Step 2, the ferrous dosing and flow rates were different for each experiment and varied over a range including 7.6 mmol/L (flow rate = 1.7 ml/min), 15.2 mmol / L (flow rate = 3.3 ml/min), 22.8 mmol / L (flow rate = 5.0 ml/min), 30.4 mmol / L (flow rate = 6.7 ml/min), and 38.0 mmol/ L (flow rate = 8.4 ml/min).

[0077] At Step 2, as described by the data of Table 4 and as shown in FIG. 3, the total selenium concentration including the selenite and selenate concentrations was reduced to less than 5 ppbw. As further shown in FIG. 3 and as described by the data of Table 4, the total selenium concentration was reduced to about 5 ppbw at a ferrous-to-selenate dosing ratio 26,550:1 moles of ferrous-to-moles of selenate. At lower ferrous-to-selenate dosing ratios, including 21,200:1 moles of ferrous-to-moles of selenate, the total selenium concentration remaining after treatment was reduced to about 15 ppbw. At a ferrous-to-selenate dosing ratios of 5,000:1 moles of ferrous-to-moles of selenate, 10,000:1 moles of ferrous-to-moles of selenate, 15,200:1 moles of ferrous-to-moles of selenate, the total selenium concentration remaining after treatment was greater than 30 ppbw.

EXAMPLE 5 (COMPARATIVE)

[0078] This Example 5 presents results from running a comparative experiment applying the single-step method for removal selenium from the waste water feed having selenium concentration.

[0079] Table 5 presents the experimental results for the total selenium concentration, including the selenite and selenate concentrations, before and after conducting the iron co addition process at a pH of 8. The co-addition technique includes adding both ferric chloride and ferrous chloride in a single step to a single reactor. The concentrations and ferrous-to- selenate dosing ratios are the same as those of Example 2.

TABLE 5

[0080] The results presented in Table 5 show that the co-addition process fails to reduce the selenate concentration below 15 ppbw at any ferrous-to-selenate dosing rate tested.

[0081] As observed from the above examples, the inventive two-step process effectively reduces the total selenium concentration, including both the individual selenate and selenite concentrations, in the treated waste water stream to less than 5 ppbw. The two-step selenium removal process surprisingly is more effective at removing selenium from a waste water stream than the comparative single-step process of co-addition of ferric chloride and ferrous chloride. The results show an unexpected correlation between the pH of the reaction products of each iron addition step of the inventive process and the reduction of selenate and selenite concentrations of the treated water. The operating pH of each of the iron addition steps contributes to achieving a selenium concentration in the resulting treated water stream that is less than 10 ppbw.

EXAMPLE 6

[0082] This Example 6 describes experiments used to determine sludge yields for the two- step process and comparative co-addition and ferric-only addition processes for selenium removal from waste water, and it describes experimental data resulting from these

experiments.

[0083] The ferric and ferrous concentrations, ferric-to- selenite dosing ratio, and ferrous-to- selenate dosing ratio are the same as those of Example 1. For the ferric-only addition process, the volume of sludge produced and settled in a one -hour period of time was tested at a pH value of 7. For the two-step iron addition process and the co-addition process, the volumes of sludge produced and settled in one -hour were tested at pH values 8 and 9.

[0084] Apart from improved selenite and selenate removal efficiencies over other iron dosing techniques, it was surprisingly found that the two-step iron addition process generated a reduced volume of sludge during a one-hour time-period as compared to co-addition and ferric-only processes.

[0085] Conventional iron addition techniques that generate higher sludge volume percentages indicate not only a larger amount of sludge to dispose of but also slower sludge settling characteristics. For example, the results from testing the inventive two-step iron addition process showed an unexpected benefit of achieving reduced sludge settling times of sludge produced. In this process, first and second sludge products were allowed to settle within first and the second separators before actually separating into component parts. The time required to achieve complete sludge settling with the inventive two-step iron addition process ranged from about 30 minutes to one -hour without the addition of any settling agents. In comparison, the time required to achieve complete sludge settling for the conventional co addition and ferric-only processes ranged from about 5 to 12 hours to achieve complete sludge settling.

[0086] The improved sludge volume and settling properties of the two-step iron addition process showed unexpected benefits over conventional methods. The reduced volume of sludge produced and quicker settling times of the inventive embodiment provide overall improvement over conventional techniques by more quickly advancing the overall selenium removal process, reducing the risk of transferring additional amounts of sludge over into the reaction products, and lowering sludge disposal amounts.

Claims

That Which is Claimed is:

1. A process for removing selenium from a waste water stream, wherein the waste water stream comprises water, a selenite concentration of selenite anions (SeCb² ), and a selenate concentration of selenate anions (Se0₄ ² ), wherein the process comprises:

(a) mixing within a first mixing zone, an aqueous ferric solution, comprising water and ferric chloride, with said waste water stream and forming a first precipitate within a first reaction product, comprising water and said first precipitate;

(b) separating, within a first separating zone, the first reaction product to yield a first water product stream having a first reduced selenite concentration and a first sludge product;

(c) mixing, within a second mixing zone, an aqueous ferrous solution, comprising water and ferrous chloride, with said first water product stream and forming a second precipitate within a second reaction product, comprising water and said second precipitate; and

(d) separating, within a second separating zone, said second reaction product to yield a second water product stream having a total selenium concentration of less than 10 ppbw and a second sludge product.

2. The process of claim 1, wherein said selenite concentration in the waste water stream is in the range of from 20 to 500 ppbw and said selenate concentration in said waste water stream is in the range of from 20 to 500 ppbw.

3. The process of claim 1, wherein said aqueous ferric solution mixed with said waste water stream has a molar concentration of ferric chloride in the range of from 0.5 mmol/L to about 20 mmol/L.

4. The process of claim 1, wherein a ferric-to-selenite dosing ratio of step (a) is in the range of from 1,000 to 5,000 mole ferric per mole selenite.

5. The process of claim 1, wherein said aqueous ferrous solution mixed with said first water product stream has a molar concentration of ferrous chloride in the range of from 5 mmol/L to about 50 mmol/L.

6. The process of claim 1, wherein a ferrous-to-selenate dosing ratio of step (c) is in the range of from 5,000 to 28,000 mole ferrous per mole selenate.

7. The process of claim 1, further comprising mixing a first pH-regulating solution with the aqueous ferric solution and said waste water stream within said first mixing zone to maintain a pH value of said first reaction product in the range of from 7 to 11.

8. The process of claim 1, further comprising mixing a second pH-regulating solution with said aqueous ferrous solution and said first water product stream within said second mixing zone to maintain a pH value of the second reaction product in the range of from 7 to 11.

9. The process of claim 1, wherein said first reaction product residence time within said first separating zone is at least 0.1 hour and said second reaction product residence time within said second separating zone is at least 0.1 hour.

10. The process of claim 1, wherein said first water product stream has a first reduced selenite concentration of less than 100 ppbw.

11. The process of claim 1, wherein said second water product stream has a reduced selenate concentration of less than 10 ppbw.

12. The process of claim 1, wherein said second water product stream comprises a second reduced selenite concentration of less than 10 ppbw.

13. The process of claim 1, wherein said total selenium concentration of said second water product stream is less than 5 ppbw.

14. The process of claim 2, wherein a ferric-to-selenite dosing ratio of step (a) is in the range of from 1,000 to 5,000 mole ferric per mole selenite.

15. The process of claim 14, wherein a ferrous-to-selenate dosing ratio of step (c) is in the range of from 5,000 to 28,000 mole ferrous per mole selenate.

16. The process of claim 15, further comprising mixing a first pH-regulating solution with the aqueous ferric solution and said waste water stream within said first mixing zone to maintain a pH value of said first reaction product in the range of from 7 to 11.

17. The process of claim 16, further comprising mixing a second pH-regulating solution with said aqueous ferrous solution and said first water product stream within said second mixing zone to maintain a pH value of the second reaction product in the range of from 7 to 11.

18. The process of claim 17, wherein said second water product stream has a reduced selenate concentration of less than 10 ppbw.

19. The process of claim 18, wherein said second water product stream comprises a second reduced selenite concentration of less than 10 ppbw.

20. The process of claim 19, wherein said total selenium concentration of said second water product stream is less than 5 ppbw.