AQUEOUS STREAMS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority to U.S. Provisional Application No. 61/597,561, filed February 10, 2012.

FIELD OF THE ART

[0002] The present disclosure relates generally to processes for the removal of metals and oxyanions from aqueous streams.

BACKGROUND

[0003] Industrial waste waters commonly include a variety of contaminants which require treatment or removal before the waste water can be discharged. Certain industrial processes, such as mining, generate waste water with toxic metals or oxyanions, including to arsenic, mercury, selenium, molybdenum, cadmium, chromium, lead, and barium.

[0004] Arsenic is a persistent, bio-accumulative toxin. Arsenic contamination is a growing concern with elevated levels being reported in the USA, Mexico, Chile, Argentina, Bangladesh, West Bengal and Brazil. The U.S. Environmental Protection Agency has set a standard of 10 ppb arsenic as the acceptable level for groundwater. Arsenic is a major component in many mining processes as it is present in high concentrations in the ore that contains metals, such as iron, gold, nickel, and cobalt.

[0005] Arsenic is stable in several oxidation states, under different redox conditions in water. However, when present in groundwater, arsenic occurs mostly in the forms of arsenite, As(III) and arsenate, As(V). As(III) is usually the predominant form in many groundwaters since it is more likely to be found in oxygen free (anaerobic) conditions. As(V) is more common in aerobic waters. In general, As(V) is more readily removed than As (III).

[0006] There are various technologies available for removing arsenic from water, including adsorption on granular iron based media; adsorption on ion exchange resins; adsorption on activated alumina; coprecipitation in iron removal plants; coagulation with alum or ferric followed by conventional filtration; coagulation with ferric followed by membrane filtration; removal by biofilms; nanofiltration; and flocculation. Many of these methods require multiple steps to pretreat or chemically reduce the contamination. BRIEF SUMMARY

[0007] Processes for removing one or more metals and/or oxyanions from an aqueous stream are provided, comprising the steps of: adjusting the pH of the aqueous stream to above about 6; adding one or more reducing agents to the aqueous stream; stirring the aqueous stream; adding one or more flocculants; and separating the reduced one or more metals and/or oxyanions from the aqueous stream.

DETAILED DESCRIPTION

[0008] Processes for removing one or more metals and/or oxyanions from aqueous streams are provided, wherein the aqueous streams are treated with one or more reducing agents to form solids of the metals and/or oxyanions. After the treatment, the solids are separated from the aqueous stream, for example by gravity settling or mechanical separation.

[0009] It has been surprisingly discovered that aqueous streams, in particular mine process waters, can be treated with one or more reducing agents, for example sodium borohydride, to remove metals and oxyanions from the aqueous streams.

[0010] In exemplary embodiments, the process for removing one or more metals and/or oxyanions from an aqueous stream comprises the steps of: adjusting the pH of the aqueous stream to above about 6; adding one or more reducing agents to the aqueous stream; agitating the aqueous stream; adding one or more flocculants; and separating the reduced one or more metals and/or oxyanions from the aqueous stream. In exemplary embodiment, the step of agitating the aqueous stream may comprise stirring or other movement of the aqueous stream. In exemplary embodiments, the process may be used to reduce the concentration of the one or more metals and/or oxyanions to below about 100 ppb, about 50 ppb, about 10 ppb, about 5 ppb, about 2 ppb, or about 1 ppb.

[0011] The expression "aqueous stream" as used herein refers to any aqueous liquid feed that contains undesirable amounts of metals or oxyanions. Exemplary aqueous streams include but are not limited to drinking water, ground water, well water, surface water, such as waters from lakes, ponds and wetlands, agricultural waters, wastewater, such as wastewater or leaching water from mining or industrial processes, geothermal fluids, water from mining processes associated with smelting, mine dewatering, tailing impoundment treatment, chemical induced leaching, flotation, autoclave, acid mine drainage, and the like. In certain embodiments, the processes can be used to remove metals and/or oxyanions from any aqueous stream containing greater than about 2.0 ppb of the metals and/or oxyanions. In one embodiment, the process is effective for treating aqueous streams containing more than 500 ppb metals and/or oxyanions. In an exemplary embodiment, the process is effective in decreasing levels of one or more metals or oxyanions to below about 100, about 10, about 5, or about 2 ppb.

[0012] Depending on the composition of the aqueous stream, the additives may change, concentrations of additives may change, and the sequence of adding the additives may change. Such changes may be determined from experience with different aqueous stream compositions.

[0013] In one embodiment, the aqueous stream is produced from a mining process, for example a smelting process, such a smelting process gold, copper, iron, nickel, silver, phosphate, coal or molybdenum; or processes associated with mine dewatering, tailing impoundment treatment, chemical induced leaching, flotation, autoclave, acid mine drainage, and the like. In exemplary embodiments, the aqueous stream comprises water and metals and/or oxyanions.

[0014] A "reducing agent," "reductant" or "reducer" is an element or compound that donates an electron to another species. In exemplary embodiments, the reducing agent may be any of a variety of reducing agents known to those of skill in the art, for example metal hydrides such as sodium borohydride, lithium aluminum hydride, and diisobutylaluminum hydride; zinc metal; iron metal; sodium sulfide; and bisulfite. In one embodiment, the reducing agent is a metal hydride. In a particular embodiment, the metal hydride is sodium borohydride. In certain embodiments, the reducing agent is not sodium sulfide. In certain embodiments, the reducing agent is not bisulfite.

[0015] The amount of reducing agent to be added to the aqueous stream is based on the amount of the aqueous stream and/or the amount of metals or oxyanions in the aqueous stream. In one embodiment, the concentration of reducing agent added to the aqueous stream is in the range of about 1000 to about 20,000 ppm, about 1000 to about 10,000 ppm, or about 2000 to about 8000 ppm. In one embodiment, the concentration of reducing agent added to the aqueous stream is about 1000, about 2000, about 3000, about 4000, about 5000, about 6000, about 7000, about 8000, about 9000, about 10,000, about 1 1,000, about 12,000, about 13,000, about 14,000, about 15,000, about 16,000, about 17,000, about 18,000, about 19,000, or about 20,000 ppm.

[0016] A variety of metals, for example heavy metals, may be removed from aqueous streams by the processes described herein, including but not limited to arsenic, mercury, selenium, molybdenum, cadmium, chromium, lead, barium, silver, other heavy metals, and mixtures thereof. In an exemplary embodiment, the metal is arsenic or a mixture comprising arsenic.

[0017] An "oxyanion" or "oxoanion" is a chemical compound of the formula A _x O _y ^z (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by many chemical elements. Exemplary oxyanions include borate, carbonate, nitrate, phosphate, sulfate, chromate, arsenate, selenate, molybdate, nitrite, phosphate, sulfite, arsenite, selenite, hypophosphite, phosphate, hyposulfite, perchlorate, perbromate, periodate, permanganate, chlorate, chromate, bromate, iodate, chlorite, bromite, hypochlorite, and hypobromite. In one embodiment, the oxyanion is arsenate. In another embodiment, the oxyanion is arsenite. In another embodiment, the oxyanion is selenate. In another embodiment, the oxyanion is selenite.

[0018] In exemplary embodiments, the metals and/or oxyanions may be a mixture of metals and/or oxyanions. In one embodiment, the metals and/or oxyanions may be a mixture comprising arsenic, arsenite and arsenate. In one embodiment, the metals and/or oxyanions may be a mixture comprising selenium, selenate and selenite.

[0019] In exemplary embodiments, the pH of the aqueous stream is adjusted by adding, for example, lime, sodium sulfide, sodium hydroxide, potassium hydroxide, other caustic substances, or mixtures thereof. In certain embodiments, the pH of the aqueous stream is adjusting adding any suitable reagent that does not comprise sodium sulfide. In the exemplary embodiments, the processes may be carried out at broad pH conditions, such as a pH of about 6 to about 12, or about 7 to about 10. In the exemplary embodiments, the pH of the aqueous stream is adjusted to above about 7, about 8, about 9 or about 10. In certain embodiments of the process, it is not necessary to adjust the pH.

[0020] In the exemplary embodiments, the processes may be carried out at temperature of about 0°C to about 100°C, or about ambient temperature to about 90°C, or about 20°C to about 90°C.

[0021] In certain embodiments, the reducing agent is a metal hydride, for example sodium borohydride.

[0022] The amount of the one or more reducing agents added during the process is any amount that is effective to reduce or stabilize at least about 50%, about 60%, about 70%, about 80%, about 90% or about 99% of the one or more metals and/or oxyanions in the aqueous stream. In exemplary embodiment, the amount of the one or more reducing agents added during the process is the amount necessary to reduce the concentration of the one or more metals and/or oxyanions to below about 100 ppb, about 50 ppb, about 10 ppb, about 5 ppb, about 2 ppb, or about 1 ppb. In certain embodiments, the reducing agent may be added to the aqueous stream in one or more doses as needed or in intervals, in a stepwise fashion, or in a continuous fashion.

[0023] In one embodiment, after adding the one or more reducing agents, the aqueous stream is stirred for a period of time from about 15 minutes to about 24 hours. In exemplary embodiments, after adding the one or more reducing agents, the aqueous stream is stirred for at least about 15 minutes, about 30 minutes, about one hour, about two hours, or about 3 hours. In certain embodiments, after adding the one or more reducing agents, the aqueous stream is stirred for any time interval that is sufficient to reduce or stabilize at least about 50%, about 60%, about 70%, about 80%, about 90% or about 99% of the one or more metals and/or oxyanions in the aqueous stream. There is no particular limit on the amount of time that the aqueous stream may be stirred after adding the one or more reducing agents.

[0024] Any suitable flocculant or mixture of flocculants may be used in the processes described herein. In certain embodiments, the one or more flocculants added to the aqueous stream comprise one or more polymer flocculants. In exemplary embodiments, the polymer flocculants may be anionic or nonionic. Any high molecular weight anionic or nonionic polymer flocculants known in the art may be used in the processes described herein. Nonlimiting examples of exemplary polymer flocculants include, for example, flocculant-grade homopolymers, copolymers, and terpolymers prepared from monomers such as (meth)acrylic acid, (meth)acrylamide, 2-acrylamido-2-methylpropane sulfonic acid, and ethylene oxide. In one embodiment, the polymer flocculant is an anionic polymers. In one embodiment, the polymer flocculant is a nonionic polymers. In one embodiment, the polymer flocculant is a mixture of anionic polymers and nonionic polymers. In an exemplary embodiment, the one or more flocculants comprises a polyacrylamide flocculant.

[0025] As used herein, the terms "polymer," "polymers," "polymeric," and similar terms are used in their ordinary sense as understood by one skilled in the art, and thus may be used herein to refer to or describe a large molecule (or group of such molecules) that contains recurring units. Polymers may be formed in various ways, including by polymerizing monomers and/or by chemically modifying one or more recurring units of a precursor polymer. A polymer may be a "homopolymer" comprising substantially identical recurring units formed by, e.g., polymerizing a particular monomer. A polymer may also be a "copolymer" comprising two or more different recurring units formed by, e.g., copolymerizing two or more different monomers, and/or by chemically modifying one or more recurring units of a precursor polymer. The term "terpolymer" may be used herein to refer to polymers containing three or more different recurring units.

[0026] In the exemplary embodiments, the dosage of the one or more flocculants can be any dosage that will achieve a necessary or desired result. In one embodiment, the dosage of the one or more flocculants is about 5 ppm to about 100 ppm; about 10 ppm to about 70 ppm; or about 20 ppm to about 50 ppm. In one embodiment, the dosage of the one or more flocculants is less than about 100 ppm, about 70 ppm, or about 50 ppm.

[0027] In certain embodiments, the process may further comprise the step of adding one or more absorbents and/or one or more coagulants.

[0028] In exemplary embodiment, the process may be repeated to further treat the aqueous stream and such subsequent treatment does not require the initial pH adjustment step. In exemplary embodiments, the process further comprises obtaining the resulting aqueous stream of the process; adding one or more reducing agents to the aqueous stream; agitating the aqueous stream; adding one or more flocculants; and separating the reduced one or more metals and/or oxyanions from the aqueous stream

[0029] In an exemplary embodiment, the process further comprises the step of adding one or more absorbents before step (d) or before the addition of the one or more flocculants. An "absorbent" or "binder" as referred to herein includes silica-based compounds, for example an inorganic silica-based polymer, a hydraulic cement, a clay- based material, cellulose, alumina-cased adsorbents, ferrohydrate adsorbents, carbon, for example carbon black, or a mixture thereof.

[0030] Exemplary hydraulic cements include Portland cement, Portland-based cement, pozzolana cement, gypsum cement, high alumina cement, slag cement, silica cement, kiln dust or mixtures thereof. In one embodiment, the hydraulic cement comprises calcium, aluminum, silicon, oxygen and/or sulfur which may set and harden by reaction with water. In one embodiment, the hydraulic cement is an alkaline cement.

[0031] Exemplary Portland cements may be those classified as class A, C, H and G cements according to American Petroleum Institute (API) specification for materials and testing for well cements. They can also be classified by ASTM CI 50 or EN 197 in classes of I, II, III, IV and V. Portland cement is the most common type of cementitious material used around the world. It consists mainly of calcium silicates and aluminates and some iron- containing phases. When mixed with water, Portland cement undergoes various hydration reactions resulting in raised pH as well as generation of new species including calcium silicate hydrates (CSHs). CSH may bind strongly to other mineral grains, resulting in a setting process. In one embodiment, the absorbent comprises Portland cement.

[0032] In the exemplary embodiments, the dosage of the one or more absorbents can be any dosage that will achieve a necessary or desired result. In one embodiment, the dosage of the one or more absorbents is about 1 to about 10,000 ppm; about 50 to about 5000 ppm; or about 100 to about 1000 ppm. In one embodiment, the dosage of the one or more absorbents is less than about 10,000 ppm, about 5000 ppm, or about 1000 ppm.

[0033] In one embodiment, the process optionally comprises the step of adding one or more coagulants before step (d) or before the addition of the one or more flocculants. A "coagulant" as referred to herein includes iron compounds or salts, for example ferric or ferrous compounds or salts; aluminum compounds or salts; hydrated lime; magnesium carbonate; a polymer that contains one or more quaternized ammonium groups or mixtures thereof. Iron coagulants include, for example, ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Aluminum coagulants include, for example, aluminum sulfate, aluminum chloride and sodium aluminate. Polymer coagulants that contain one or more quaternized ammonium groups include, for example acryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyltrimethylammonium chloride, methacrylamidopropyltrimethylammonium chloride, and acrylamidopropyltrimethylammonium chloride.

[0034] In the exemplary embodiments, the dosage of the one or more coagulants can be any dosage that will achieve a necessary or desired result. In one embodiment, the dosage of the one or more coagulants is about 1 to about 15 times the amount of the metals and/or oxyanions by mass (e.g. Fe:As mass ratio). In one embodiment, the dosage of the one or more coagulants is less than about 15 times the amount of the metals and/or oxyanions by mass.

[0035] According to the embodiments, the separation step may be accomplished by any means known to those skilled in the art, including but not limited to gravity settling, centrifuges, hydrocyclones, decantation, filtration, thickeners or another mechanical separation method.

[0036] In the exemplary embodiments, the reduced metals and/or oxyanions may be handled or processed in any manner as necessary or desired. In one embodiment, the reduced metals and/or oxyanions should be handled in compliance with governmental regulations. In some embodiments, the reduced metals and/or oxyanions may be disposed of, sent to a landfill, or when solids are a concentrated source of minerals, the solids may be used a raw materials or feed to produce compounds for commercial products.

[0037] In certain embodiments, the process further comprises adding one or more sulfur-containing compounds. The sulfur-containing compound can be, for example, a sulfide, a polysulfide, hydrogen sulfide, dimethylthiolcarbamate, diethylthiolcarbamate, sodium sulfide, sodium thiosulfate, calcium polysulfide, and mixtures thereof. The amount of the sulfur-containing compound to be added to the aqueous stream is dependent on the amount of metal or oxyanion present in the aqueous stream. In one embodiment, the amount of the one or more sulfur-containing compound to be added is in the range of about 1.0 mole to about 4.0 moles, about 1.0 mole to about 3.0 moles, or about 1.0 mole to about 2.0 moles, per mol of metals or oxyanions.

[0038] In a particular embodiment, the processes may be used to reduce or eliminate metals or oxyanions from waste water streams in mining processes.

[0039] In exemplary embodiments, the processes may be used to form a salt of the metal or oxyanion, wherein the salt has very low solubility in the aqueous stream or in water.

[0040] The following examples are presented for illustrative purposes only, and are not intended to be limiting.

EXAMPLES

[0041] Example 1. Batch Test for Arsenic Removal

[0042] The batch experiments were conducted on a laboratory scale and the objective of these tests were to: (1) achieve arsenic concentrations lower than 0.2 ppm in the liquid phase after treatment; (2) decrease the amount of solid residue (secondary waste) generated by the treatment and at the same time increase the arsenic concentration within the residue; and (3) determine reagent consumption to achieve feasible treatment costs.

[0043] These experiments evaluate to ability of certain reducing agents to precipitate arsenic or arsenic compounds from aqueous solutions.

[0044] The experiments were performed with 500 mL of raw process water (e.g. water from a gold smelting process), characterized by Inductively Coupled Plasma (ICP) chemical analysis. The samples were agitated in a 1000 mL beaker with a reagent to adjust the pH (e.g. lime). Once the desired pH was obtained (e.g. pH of 6-8) different amounts of a reducing agent were added (e.g. 6000 ppm of 12% sodium borohydride solution) with agitation in order to determine the optimal dosage. The solution was then mixed with low agitation for a given time (for example, about 2 to about 24 hours, or preferably about 3 hours). After that, flocculant (e.g. an acrylamide flocculant) was added and the solution was kept at rest for precipitation. After the required resting time (for example, about 1 to about 60 minutes), the supernatant of the test with the best dosage of the reducing agent was collected (e.g. by filtration) and kept to rest overnight. Again, the supernatant was submitted to the reduction treatment and let to rest overnight. Analyzing the residue and the supernatant of this last treatment showed results that indicated arsenic levels in the liquid phase were compatible with discharge limits determined by TCLP analysis. The TCLP analysis is a standard test developed by the United States Environmental Protection Agency (EPA). This test determines which of the contaminants identified by EPA are present in the leachate and their concentrations; it simulates landfill conditions. The TCLP extract containing in arsenic concentration has to be less than about 5 ppm. Concerning to the solid sediment residue, the amount of it has been decreased and its arsenic grade increased, both satisfactory.

[0045] A Thermo Scientific leap 6000 Series Inductively Coupled Plasma Atomic Emission Spectrometer was used for all measurements, and arsenic content present in the raw waste water was 810 to 1600 ppm.

[0046] In order to optimize the reduction agent consumption, some additional tests were conducted by adding the reduction agent in a stepwise manner to the arsenical liquor.

[0047] These results support that arsenic levels in mine process water can be reduced from 1200 ppm to less than 100 ppb. It is also noted that this process generates less than about 5% of the contaminated sludge that must be stored in a chemical landfill.

[0048] Example 2. Batch test for Arsenic and Selenium Removal

[0049] The batch experiments were conducted on a laboratory scale and the objective of these tests were to: (1) achieve arsenic concentrations lower than 0.2 ppm in the liquid phase after treatment; (2) achieve selenium concentrations lower than 0.005 ppm in the liquid phase after treatment; (3) decrease the amount of solid residue (secondary waste) generated by the treatment and at the same time increase the arsenic concentration within the residue; and (4) determine reagent consumption to achieve feasible treatment costs.

[0050] These experiments evaluate to ability of certain reducing agents to precipitate arsenic, arsenic compounds, selenium and selenium compounds from aqueous solutions.

[0051] The experiments were performed with 500 mL of raw process water (e.g. water from a gold smelting process), characterized by ICP chemical analysis. Table 1 provides the concentration of selected elements in the water samples, and the pH of each water sample, prior to treatment.

[0052] Table 1. Raw Waste Water Chemical Characterization by ICP Analysis

[0053] Pretreatment: The samples were agitated in a 1000 mL beaker with a reagent to adjust the pH (e.g. lime or a reagent with a pH greater than 10.5). A flocculant was also added as part of this pretreatment step, for example 20 ppm of a polymeric acrylamide flocculant and the sample was allowed to rest for a period of time (e.g. 8 hours).

[0054] First treatment: Different amounts of a reducing agent were added (e.g. 100, 500, or 1000 mg/L sodium borohydride) to the samples with agitation. The solution was then mixed with low agitation for a given time (for example, about 2 to about 24 hours). After that, 20 ppm flocculant (e.g. an acrylamide flocculant such as Superfloc® A 130 HMW from Kemira Oyj) was added and the solution was kept at rest (sedimentation) for precipitation for a period of time (e.g. 2 hours or overnight). [0055] Second treatment: Certain samples were submitted to a further reduction treatment in which 100 mg/L of sodium borohydride and 20 mg/L of a flocculant were added. The samples were subsequently let to rest (sedimentation) for a period of time (e.g. 2 hours).

[0056] The results of the treatments, as well as additional information and measurements from various points in the treatment process, are shown in Table 2 below. Residue mass is the solid mass after the second treatment. The total residue mass is the mass generated from the combination of treatment steps.

[0057] Table 2. Results

[0058] The ICP Detection limit for arsenic or selenium is in the range of >0.1 and < 1 μg/L (Thermo Electron Corporation Technical Note #40839). [0059] Analyzing the residue and the supernatant of this treatment showed results that indicated arsenic levels in the liquid phase were compatible with discharge limits determined by TCLP analysis. The TCLP analysis is a standard test developed by the United States Environmental Protection Agency (EPA). This test determines which of the contaminants identified by EPA are present in the leachate and their concentrations; it simulates landfill conditions. The TCLP extract containing in arsenic concentration has to be less than about 5 ppm. Concerning to the solid sediment residue, the amount of it has been decreased and its arsenic grade increased, both satisfactory. The amount of selenium in the sludge has increased as a result of the treatment.

[0060] A Thermo Scientific leap 6000 Series Inductively Coupled Plasma Atomic Emission Spectrometer was used for all measurements.

[0061] These results support that arsenic levels in mine process water can be reduced from about 1200 ppm to less than about 1 ppm. It is also noted that this process generates less than about 5% of the contaminated sludge that must be stored in a chemical landfill.

[0062] In the preceding specification, various exemplary embodiments have been described. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the exemplary embodiments as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.