CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61/513,825, filed August 1, 2011. This application is a continuation-in-part of U.S. Patent Application Serial No. 13/223,045, filed August 31, 2011, and entitled "Electrochemical Process to Recycle Aqueous Alkali Chemicals Using Ceramic Ion Conducting Membranes," which is a divisional of U.S. Patent Application Serial No. 12/062,458, filed April 2, 2008, and entitled "Electrochemical Process to Recycle Aqueous Alkali Chemicals Using Ceramic Ion Conducting Membranes." These patent applications are expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] Alkali aluminate compounds are obtained in various industrial reactions. For example, sodium aluminate is formed by the reaction of aluminum metal with sodium hydroxide as follows:

[0003] 2 Al + 2 NaOH + 6 H ₂ 0→ 2 NaAl(OH) ₄ + 3 H ₂

[0004] Alkali aluminate is formed by the neutralization of aluminum oxide (alumina) with a base, such as sodium hydroxide, as follows:

[0005] A1 ₂ 0 ₃ + 2 NaOH + 3 H ₂ 0→ 2 NaAl(OH) ₄

[0006] It has proven difficult to recover valuable aluminum and alkali metal compounds from industrial waste streams containing alkali aluminate. It would be an advancement in the art to provide apparatus and methods to produce and recover aluminum hydroxide from alkali aluminate based aqueous streams.

BRIEF SUMMARY OF THE INVENTION

[0007] The present invention provides methods of producing and recovering aluminum hydroxide and alkali hydroxide from alkali aluminate based aqueous streams. Alkali aluminate may exist in different forms. For instance, an anhydrous form is represented as MA10 ₂ or M ₂ A1 ₂ 0 ₄ , wherein M is an alkali metal, such as lithium, sodium, or potassium. Alkali aluminate may exist in a hydrated form as MAl(OH) ₄ . A hydrated aluminate ion may be represented as [Al(OH) ₄ ] ^~ . The present invention further provides a method of converting alkali aluminate into alkali hydroxide and aluminum hydroxide. [0008] The disclosed methods are enabled by the use of an alkali ion conductive membrane in an electrolytic cell. The alkali ion conductive membrane may include a chemically stable ionic-selective ceramic membrane. A layered composite of a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane may also be used to take advantage of the chemical stability of the ionic-selective polymer and the high alkali- ion selectivity of cation-conductive ceramic materials.

[0009] The electrolytic cell includes an alkali ion conductive membrane configured to selectively transport alkali ions. The membrane separates the electrolytic cell into an anolyte compartment configured with an electrochemically active anode and a catholyte compartment configured with an electrochemically active cathode.

[0010] The alkali aluminate containing aqueous solution may be introduced into the anolyte compartment. Additional reaction byproducts may be present in the anolyte compartment, including oxygen or hydroxide. An anolyte solution containing alkali aluminate compounds is introduced into the anolyte compartment. The alkali aluminate compounds may comprise hydrated alkali aluminate, represented as ΜΑ1(ΟΗ) ₄ , M is an alkali metal. Non-limiting examples of alkali aluminate compounds include sodium aluminate (NaAl(OH) ₄ ), potassium aluminate (KAl(OH) ₄ ), and lithium aluminate (LiAl(OH) ₄ ). Water or an alkali base solution is introduced into the catholyte compartment.

[0011] In a disclosed embodiment, an electric current is applied to the electrolytic cell to produce hydrogen ions at the anode in the anolyte compartment according to the following reaction:

[0012] Anode: H ₂ 0→ 2e ^" + ½ 0 ₂ + 2 H ⁺

[0013] The existence of hydrogen ions lowers the pH within the anolyte compartment. The available hydrogen ions react with the alkali aluminate to form aluminum hydroxide as follows:

[0014] H ⁺ + MAl(OH) ₄ → Al(OH) ₃ + H ₂ 0 + M ⁺

[0015] The free alkali ions (M ⁺ ) are transported from the anolyte compartment to the catholyte compartment through the alkali ion conductive membrane. The removal of alkali ions from the anolyte compartment further facilitates formation of aluminum hydroxide.

[0016] In another disclosed embodiment, the anolyte solution may further comprise alkali hydroxide. In such cases, an electric current applied to the electrolytic cell may produce oxygen at the anode in the anolyte compartment according to the following reaction:

[0017] Anode: 4 OH ^" → 2 H ₂ 0 + 0 ₂ + 4 e ^" [0018] Alternatively, in such cases where the anolyte solution further comprises alkali hydroxide, available hydrogen ions may also neutralize hydroxide ions in addition to reacting with alkali aluminate.

[0019] Water is decomposed in the presence of alkali ions in the catholyte compartment to form hydroxide ions (OH ) and hydrogen gas according to the following reaction:

[0020] Cathode: 2 H ₂ 0 + 2 e ^" → H ₂ + 3 OH ^"

[0021] The influence of the electric potential causes free alkali ions to pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment. The alkali ions combine with hydroxide ions to form alkali hydroxide as follows:

[0022] 2 M ⁺ + 2 OH ^" → 2 MOH

[0023] Aluminum hydroxide and unreacted alkali aluminate are removed from the anolyte compartment. Cooling from processing operating conditions due to alkali metal separation causes aluminum hydroxide to precipitate. It is recovered by conventional solid / liquid separation techniques, including, but not limited to, filtering, centrifuging, etc. The recovered aluminum hydroxide can be further processed, if desired, or used in other industrial processes. In one non-limiting example, aluminum hydroxide is heated to form alumina (Al ₂ 0 ₃ ) as follows:

[0024] 2 Al(OH) ₃ → A1 ₂ 0 ₃ + 3 H ₂ 0

[0025] The supernate following removal of precipitated aluminum hydroxide may be recycled and added to the anolyte feed for further processing with the electrolytic process to separate sodium and aluminum products.

[0026] The alkali hydroxide solution produced in the catholyte compartment may be removed for use in other industrial processes.

[0027] To increase the efficiency of the apparatus and method, hydrogen gas produced in the catholyte compartment may be collected or used to generate power for use in the process. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0028] In order that the manner in which the above -recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: [0029] Figure 1 provides a schematic view of a two compartment electrolytic cell with an apparatus and process for separating alkali metal ions from alkali metal salts of alkali aluminate; and a method for separation of aluminum hydroxide and feeding of the supernate back with anolyte feed of the electrolytic process.

[0030] Figure 2 is a graph of current versus voltage to drive sodium across a sodium conductive membrane in a two compartment electrolytic cell to separate sodium from a solution containing sodium aluminate.

[0031] Figure 3 is a photograph that shows the formation of aluminum hydroxide precipitate from separation of sodium from the alkali aluminate in anolyte solution.

[0032] Figure 4 is the analysis of the aluminum hydroxide precipitate separated from the process by X-ray diffraction method.

[0033] Figure 5 is a micrograph from a scanning electron microscope showing morphology of the precipitate material formed.

[0034] Figure 6 is a graph of current versus voltage to drive sodium across a sodium conductive membrane in a two compartment electrolytic cell in multiple batches of operation with alkali hydroxide and alkali aluminate based solution to separate sodium and aluminum.

[0035] Figure 7 is a photograph that shows the formation of aluminum hydroxide precipitate from separation of sodium from the alkali aluminate in anolyte solution.

[0036] Figure 8 show a potential method to separate the precipitate product and the method to feed the permeate solution with the anolyte to the electrochemical cell for further separation of sodium and aluminum.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and cells of the present invention, as represented in Figure 1 , is not intended to limit the scope of the invention, as claimed, but is merely representative of embodiments within the scope of the invention.

[0038] Figure 1 illustrates a general schematic view for an apparatus and method for separating aluminum hydroxide and alkali metal ions from an alkali aluminate and hydroxide solution within the scope of the present invention. The apparatus and process for separating alkali metal ions includes an electrolytic cell 100. [0039] The electrolytic cell 100 uses an alkali ion conductive membrane 112 that divides the electrochemical cell 100 into two compartments: an anolyte compartment 114 and a catholyte compartment 116. An electrochemically active anode 118 is housed in the anolyte compartment 114 where oxidation reactions take place, and an electrochemically active cathode 120 is housed in the catholyte compartment 116 where reduction reactions take place. The alkali ion conductive membrane 112 selectively transfers alkali ions (M ⁺ ) 122 from the anolyte compartment 114 to the catholyte compartment 116 under the influence of an electrical potential 124. In one non-limiting embodiment, the membrane 112 may comprise an ionic-selective ceramic membrane stable in the environment of the anolyte and catholyte compartments. In another non-limiting embodiment, the membrane 112 may include a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

[0040] The electrolytic cell 100 is operated by feeding an anolyte solution feed stream 126 into the anolyte compartment 114. The anolyte solution feed stream 126 comprises an aqueous solution of alkali aluminate. The anolyte solution feed stream may also comprise alkali hydroxide.

[0041] A catholyte feed stream 128 comprising water or a low concentration alkali salt, is fed into the catholyte compartment 116. During operation, the source of alkali ions in the catholyte compartment 116 may be provided by alkali ions 122 transporting across the alkali ion conductive membrane 112 from the anolyte compartment 114 to the catholyte compartment 116.

[0042] The anode 118 may be fabricated of various materials, including those discussed below. In one non- limiting embodiment, the anode 118 is fabricated of Nickel, Iron-Nickel- Cobalt and stainless steel chemistries. The cathode 120 may also be fabricated of various materials, including those discussed below. In one non-limiting embodiment, the cathode 120 is fabricated of nickel/stainless steel.

[0043] Under the influence of electric potential 124, electrochemical reactions take place at the anode 118 and cathode 120. Non-limiting examples of such reactions are shown below:

[0044] Anode: H ₂ 0→ 2e ^" + ½ 0 ₂ + 2 H ⁺

[0045] Cathode: 2 H ₂ 0 + 2e ^~ → H ₂ + 3 OH ^"

[0046] The available hydrogen ions react with the alkali aluminate in the anolyte compartment to form aluminum hydroxide as follows:

[0047] H ⁺ + MAl(OH) ₄ → Al(OH) ₃ + H ₂ 0 + M ⁺ [0048] The operating temperature within the anolyte compartment in one embodiment is at least 40°C and higher. A higher operating temperature will support a higher aluminum hydroxide solubility in the bulk anolyte solution. It is desirable to maximize the aluminum hydroxide solubility so that a maximum of alkali metal ions may pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment to the point where the aluminum hydroxide is close to saturation, saturated or super saturated. The anolyte solution may then be removed from the anolyte compartment and cooled to promote precipitation of the aluminum hydroxide.

[0049] The influence of the electric potential causes free alkali ions (M ⁺ ) to pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment. The removal of alkali ions from the anolyte compartment further facilitates formation of aluminum hydroxide. The alkali ions combine with hydroxide ions to form alkali hydroxide solution as follows:

[0050] 2 M ⁺ + 2 OH ^" → 2 MOH

[0051] A catholyte exit stream 133 permits removal of the alkali hydroxide solution from the catholyte compartment for use in other chemical processes. A hydrogen gas vent 132 permits hydrogen gas produced in the catholyte compartment 116 to be vented and collected from the catholyte compartment 116. The hydrogen gas may provide fuel to an alternative energy generating process, such as a polymer electrolyte membrane, also known as proton exchange membrane, (PEM) fuel cell or other device known to one of ordinary skill in the art for energy generation. This may help offset the energy requirements to operate the electrolytic processes. The hydrogen gas may be used for chemical processes known to one of ordinary skill in the art. An oxygen gas vent 134 permits oxygen gas produced in the anolyte compartment 114 to be vented and collected from the anolyte compartment 114. The oxygen may be used for chemical processes known to one of ordinary skill in the art.

[0052] Anolyte exit stream 136 is removed from the anolyte compartment 114 for further processing. Stream 136 contains aluminum hydroxide. It may also contain unreacted alkali aluminate, alkali hydroxide, or other chemical moieties. Stream 136 may optionally be fed to a separator 138. In separator 138, the contents of stream 136 are cooled to cause precipitation of aluminum hydroxide 140 which may be removed by any suitable mechanical separation process. Such mechanical separation processes include, but are not limited to, centrifuge, screen press, belt press, and other industrial sedimentation, separation or filtration processes known in the art. A supernate stream 142 connected to separator 138 may recycle at least a portion of the supernate solution containing sodium and aluminum compounds back to the anolyte feed stream 126. Recycling the supernate into the electrolytic cell 100 permits further removal of sodium and aluminum compounds.

[0053] The anolyte compartment may optionally contain a temperature control unit 144 to control the operating temperature of the anolyte compartment. The operating temperature in one embodiment is at least 40°C or higher to increase the aluminum hydroxide solubility in the bulk solution. A higher aluminum hydroxide solubility allows more alkali metal ions to be removed from the anolyte compartment and transported across the alkali ion conductive membrane into the catholyte compartment where it may be recovered as alkali hydroxide.

[0054] Electrode materials useful in the methods and apparatus of the present invention are electrical conductors and are generally substantially stable in the media to which they are exposed. Any suitable electrode material or combination of electrode materials, known to one of ordinary skill in the art may be used within the scope of the present invention. Non- limiting examples of some electrode materials include titanium coated with advanced metal oxides, nickel, Kovar (Ni-Fe-Co), stainless steel, carbon steel, and graphite.

[0055] In some specific embodiments, the anode material may include at least one of the following: dimensionally stable anode, nickel, and cobalt, and nickel tungstate, nickel titanate, metal oxides based on titanium, stainless steel, lead, lead dioxides, graphite, tungsten carbide and titanium diboride. In some specific embodiments, the cathode material may include at least one of the following: nickel, cobalt, platinum, silver, alloys such as titanium carbide with small amounts (in some instances up to about 3 weight ) of nickel, FeAl ₃ , N1AI ₃ , stainless steel, perovskite ceramics, and graphite. In some embodiments, the electrodes may be chosen to maximize cost effectiveness by balancing the electrical efficiency of the electrodes against their cost.

[0056] The electrode material may be in any suitable form within the scope of the present invention, as would be understood by one of ordinary skill in the art. In some specific embodiments, the form of the electrode materials may include at least one of the following: a dense or porous solid-form, a dense or porous layer plated onto a substrate, a perforated form, an expanded form including a mesh, or any combination thereof.

[0057] In some embodiments of the present invention, the electrode materials may be composites of electrode materials with non-electrode materials, where non-electrode materials are poor electrical conductors under the conditions of use. A variety of insulative non-electrode materials are also known in the art, as would be understood by one of ordinary skill in the art. In some specific embodiments, the non-electrode materials may include at least one of the following: ceramic materials, polymers, metal, and/or plastics. These non- electrode materials may also be selected to be stable in the media to which they are intended to be exposed.

[0058] Other variations, including variations of electrode material, shape, and in some instances, placement could be made within the scope of the invention by one of ordinary skill in the art.

[0059] The alkali ion conductive membrane 112 utilized in the processes and apparatus of the present invention are alkali cation-conductive, and physically separate the anolyte solution from the catholyte solution. The membrane 112 includes a chemically stable ionic- selective ceramic membrane. Such membranes may be stable in a wide range of pH conditions. The membrane 112 may include a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

[0060] In one embodiment, the alkali ion conductive membranes conduct lithium ions, sodium ions, or potassium ions. It may be advantageous to employ membranes with low or even negligible electronic conductivity, in order to minimize any galvanic reactions that may occur when an applied potential or current is removed from the cell containing the membrane. In some embodiments of the present invention it may be advantageous to employ membranes that are substantially impermeable to at least the solvent components of both the catholyte and anolyte solutions.

[0061] In some embodiments of the alkali ion conductive membrane of the present invention, the ceramic membrane may not be substantially influenced by scaling, fouling or precipitation of species incorporating divalent cations, trivalent cations, and tetravalent cations; or by dissolved solids present in the solutions.

[0062] For those embodiments utilizing an alkali ion conductive ceramic membrane, the alkali ion conductive ceramic materials are configured to selectively transport alkali ions. They may be a specific alkali ion conductor. For example, the alkali ion conductive ceramic membrane may be a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li. The alkali ion conductive ceramic membrane may comprise a material having the formula Mi _+x M ^I ₂ Si _x P3_ _x Oi ₂ where 0<x<3, where M is selected from the group consisting of Li, Na, K, or mixture thereof, and where M ¹ is selected from the group consisting of Zr, Ge, Ti, Sn, or Hf, or mixtures thereof; materials of general formula Nai _+z L _z Zr ₂ - _z P30i ₂ where 0<z<2.0, and where L is selected from the group consisting of Cr, Yb, Er, Dy, Sc, Fe, In, or Y, or mixtures thereof; materials of general formula M ⁿ sRESi ₄ 0i ₂ , where M ⁿ may be Li, Na, or any mixture thereof, and where RE is Y or any rare earth element. [0063] Several examples are provided below which discuss specific embodiments within the scope of the invention. These embodiments are exemplary in nature and should not be construed to limit the scope of the invention in any way.

[0064] Example 1

[0065] A solution containing 5.64 molarity of NaOH in solution sodium aluminate waste stream was heated to 40°C as the anolyte. The anode was Kovar (Fe-Ni-Co) and the cathode was Kovar. The cell was operated in a batch mode of operation at a current density of 75 mA per sq.cm. of membrane area. The initial catholyte was 1M NaOH. Figure 2 is a plot which presents the sodium transfer current-versus voltage to drive sodium across the two compartment cell to thereby separate sodium from sodium aluminate.

[0066] The voltage remained between 4 to 5 volts during the entire duration for majority of the test. It should be noted that the cell was operated for a known duration in batch mode to establish cell performance only. A total of 82.7 % of sodium was separated from the sodium aluminate sample in this test as determined by ICP analysis. Figure 3 shows samples collected at different level of sodium separation with membrane cell and formation of aluminum hydroxide. The level of sodium separation ranged from 37.3% to 82.7%.

[0067] The average power consumption of this cell was 1.31 kWhr/lb of NaOH produced (or 2615 kWhr/ton NaOH produced (on dry basis). We have demonstrated with the two compartment cell to make sodium hydroxide in the catholyte. The precipitated material in the anolyte was removed after the sodium transfer and was analyzed by Scanning electron microscope SEM/EDAX.

[0068] Table 1 shows analysis of the sodium aluminate based samples before and after processing within the electrolytic cell containing the sodium conductive ceramic membrane. Table 1 specifically shows the sodium and aluminum analysis to determine separation of sodium and aluminum from the alkali aluminate solution before and after electrolysis and aluminum hydroxide precipitation.

[0069] Table 1

[0070] Figure 4 shows X-Ray Diffraction (XRD) analysis of the precipitate formed during testing to determine its composition. The precipitate was identified as aluminum hydroxide (Al(OH) ₃ ), also known as gibbsite.

[0071] Figure 5 show the SEM image of the precipitated aluminum hydroxide material. The aluminum hydroxide appears to form 5-10μιη platelets.

[0072] Example 2

[0073] A NaSICON membrane was assembled in a two-compartment cell configuration and operated an in electrochemical cell with anolyte and catholyte solutions. Operated at constant current density of 75 mA/cm ² , several batch tests were conducted to demonstrate the approach to produce sodium hydroxide and aluminum hydroxide from the waste sodium aluminate based sample. The electrolytic cell was operated for about 20 hours at 40° C. The initial and final anolyte and catholyte solutions were submitted for sodium mass balance analysis to determine the sodium concentration. The average power consumption to make NaOH was determined from the sodium mass balance analysis results.

[0074] Figure 6 is a plot which presents the sodium transfer at constant current, the voltage is the potential required to drive sodium across the two compartment cell operated in batch mode as a function of time to thereby separate sodium from sodium aluminate in multiple batch testing. The voltage remained between 4 to 5 volts during the duration of test for each independent batch operation with the fresh waste sample solution. It should be noted that the cell was operated for a known duration to establish cell performance only. The amount of sodium separated from the sodium aluminate sample by ICP analysis ranged from 72.7% to 85.0%. The average power consumption of this cell was 1.21 kWhr/lb NaOH produced (or 2416 kWhr/ton NaOH produced on dry basis).

[0075] Figure 7 shows samples taken from each batch of test to show making of aluminum hydroxide in the anolyte after separation of sodium from the stream during operation in electrochemical cell.

[0076] A method to separate sodium aluminate precipitate from the anolyte as it forms during sodium separation from sodium aluminate anolyte stream in an electrochemical cell is presented in Figure 8. The scheme shows one of the several methods which can be followed to separate aluminum (aluminum hydroxide based precipitates) and to recycle the supernate solution containing additional sodium and aluminum compounds back to anolyte solution feed.

[0077] The process flow diagram in Fig. 8 outlines the one-step sodium removal from sodium aluminate process stream and simultaneous production of sodium hydroxide. The major steps in the process are described below. The Sodium Aluminate Process Stream is fed to the Ceramatec Electrochemical Cell from the Anolyte Feed Tank through a Heat Exchanger at a required temperature as the anolyte solution. On passing through the Ceramatec Electrochemical Cell, sodium ions are transferred across the ion exchange membrane from the process stream and passed into the aqueous sodium hydroxide solution which exits the catholyte compartment. The anolyte solution from the Ceramatec Electrochemical Cell is then sent through a Cooling Exchanger to an Aluminum Separation Vessel to remove precipitated aluminum hydroxide solids. The solid rich solution from the Aluminum Separation Vessel is removed while the solid lean solution, labeled as Permeate Stream is returned to the Anolyte Feed Tank for recirculation. At the start of the process, a certain concentration of aqueous sodium hydroxide solution is fed to the Ceramatec Electrochemical Cell from the Catholyte Feed Tank through a Heat Exchanger at a required temperature as the catholyte. On passing through the Ceramatec Electrochemical Cell, the solution is enriched with sodium ions (sodium hydroxide) by their transfer through the sodium selective membrane from the anolyte solution. The enriched solution is received back into the Catholyte Feed Tank which is purged with nitrogen to remove the hydrogen from the Tank. Water is continuously added to the Catholyte Feed Tank to keep the concentration of sodium hydroxide constant. Aqueous sodium hydroxide is continuously removed from the Catholyte Feed Tank as the product.

[0078] While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.