WO2014042785A1

WO2014042785A1 - Process for deoxygenation and chemical reduction of spent in-situ leach water from uranium mining

Info

Publication number: WO2014042785A1
Application number: PCT/US2013/053881
Authority: WO
Inventors: Anthony P. Tirio; Phil Fatula; Bjoern DINGES
Original assignee: Lanxess Sybron Chemicals, Inc.
Priority date: 2012-09-11
Filing date: 2013-08-07
Publication date: 2014-03-20

Abstract

A process for in situ de-oxygenation of water from uranium mining, in which water in a spent deposit is extracted, de-oxygenated and returned to the formation; and wherein the method employees the use of a weak base anion ion exchange resin doped with palladium metal as catalyst for the de-oxygenation reaction.

Description

PROCESS FOR DEOXYGENATION AND CHEMICAL REDUCTION OF SPENT IN-SITU LEACH WATER FROM URANIUM MINING

This application claims priority to provisional U.S. Application Nos. 61/699,475, filed September 11, 2012, entitled PROCESS FOR DEOXYGENATION AND CHEMICAL REDUCTION OF SPENT IN-SITU LEACH WATER FROM URANIUM MINING, incorporated herein by reference.

Conventional mining where the ore of interest is excavated along with some surrounding earth materials is subject to a myriad of personal and environmental hazards. Many examples of mine cave-ins and deaths of miners have been publicized. Environmentally, problems with surface degradation due to mining techniques and disposal of tailings add to the problems of conventional mining.

As an alternative to conventional mining, in-situ processes have been developed to extract valuable ores. As the name entails, the ore extraction occurs in place below the ground surface with very little disturbance above ground. As depicted in Figure I, in-situ recovery of uranium involves pumping a solution, called a lixiviant, into the uranium ore pocket to dissolve the ore and carry it back to the surface for uranium recovery. ("Generic Environmental Impact Statement for In-Situ Leach Uranium Milling Facilities", Chapter 2, US Nuclear Regulatory Commission, NUREG- 191G, May 2009)

Early ISL operations used sulfuric acid based lixiviants to dissolve the uranium from the ore and allow it to flow back to the surface. While very effective for dissolving uranium, sulfuric acid also dissolves other unwanted metals and minerals. Modern lixiviant is made up of a water solution of sodium bicarbonate and carbonate enhanced with gaseous oxygen to dissolve the uranium oxide trapped under ground. The kinetics of the dissolution process has been found to be linearly dependent upon the concentration of dissolved oxygen. (Grandstaff, D.E., "A Kinetic Study of the Dissolution of Uraninite", Economic Geology, v. 71, pp. 1493-1506) In alkaline recovery of uranium, the following chemical reactions define the major conceptualized steps to dissolve uranium oxide compounds. Since uraninite, represented as U02 below, is a complex composite of minerals, the steps as shown may be considered as one for illustration purposes.

(1) uo₂ +½<¾ <→ uo₃

(2) E70₃ + 3NaC0₃ + H₂0 <→· Na₄U0₂ (C0₃ )₃ + INaOH

Above ground, the water soluble uranyl carbonate can be recovered from the rich stream by adsorption on highly selective weak base anionic resin. The post recovery lean stream is re-oxygenated and re-carbonated before being injected again for further use.

As some point during the extraction process, usually two or three years after initial operation, the underground deposit is depleted to the point of being uneconomical for further extraction. If oxygenated carbonate lixiviant is allowed to remain underground in the deposit, there is a possibility of bleeding uranyl carbonate into surrounding ground waters. To mitigate this situation, it has been found to be beneficial to reverse the process shown in equations (1) and (2) above. Water in the spent deposit can be extracted, de-oxygenated and returned to the formation to stop the solution process. This is depicted in Figure 2. The method and catalyst for deoxygenation is the subject of this invention.

Oxygen can be removed from water by several different methods. The removal methods can be classified as either physical or chemical processes. Two examples of physical deoxygenation are vacuum degassing and reverse osmosis. One example of chemical deoxygenation is sulfite addition to bind elemental oxygen and produce a soluble salt. Each of the three examples is found to be inferior to the catalytic destruction of oxygen of the subject patent.

Vacuum degassing relies on the relationship described by Henry's Law. Henry's Law states that the solubility of an ideal gas in a solution is directly propoitional to the partial pressure of that gas in the vapor space in equilibrium with the solution. The proportionality is affected by the temperature of the solution with higher temperatures favoring lower gas solubility. It is therefore readily apparent that the amount of gas dissolved in a solution can be reduced by applying heat and vacuum to the system to draw the gas out of the solution. It can also be understood that by Dalton's Law and material balance considerations that the vapor space above the solution to be deoxygenated is predominantly water vapor. Water removed by vacuum will be replaced by water evaporating from the solution to be deoxygenated in turn removing heat from the solution. If the heat is not replaced, the solution will auto refrigerate resulting in lower temperatures and greater gas solubility (see Henry's Law). Hence, we see that vacuum deoxygenation can be energy intensive with demands for mechanical energy to supply vacuum and thermal energy to maintain system temperature.

Reverse osmosis type membranes can be used to remove dissolved oxygen from clean water streams. The contactors are made with micro porous, hollow fiber hydrophobic membranes separating the water phase from a bulk gas phase. The process water passes over the membranes while the gas phase across the membrane is either under vacuum or exposed to a sweep gas to provide a low partial pressure of oxygen. The gas passes through the hydrophobic membrane which concurrently rejects water. It can readily be seen that the membranes experience massive fouling and blockage when exposed to the mineral and solids laden lixiviant used in uranium recovery. There has been limited success for using membrane deaerators in this service even after extensive filtration and scale inhibition.

Sodium sulfite, widely known as an oxygen scavenger, can be used to chemically remove dissolved oxygen from water.

(3) 2N ₂S0₃ + 0₂→ 2Na₂SO_t

Although the theoretical demand of sulfite is 7.88 weight units per weight unit of dissolved oxygen, normal operation requires approximately 10 weight units of sulfite per weight unit of oxygen to be removed. The presence of heavy metal cations tends to increase the reaction rate for oxygen removal while organic acids will retard the reaction. While sodium sulfite is an effective oxygen scavenger in remediation of in- situ uranium mining operations, the cost can be prohibitive.

Hydrazine can also be used but will be even less cost effective than sulfite in uranium in-situ mining remediation.

Currently, an investigation is being performed at the Uranium Resources, Inc Kings ville Dome, Texas in situ uranium extraction site on stabilizing the soluble hcxavalent uranium. This experimental process, being developed in conjunction with Texas A&M University at Kingsville, adds elemental hydrogen in an aqueous injection stream to initiate a reducing environment below ground and convert ionic uranium to insoluble U02. (Hall, S., "Groundwater Restoration at Uranium In-Situ recovery Mines, South Texas Coastal Plain", USGS Open-File Report 2009-1143, p 28) Without a nobel metal catalyst or extremely high pressures and temperatures, it is unlikely in the aqueous environment that the injected elemental hydrogen will react with the dissolved oxygen below grade to halt the oxidative dissolution of uranium. Results are still being evaluated.

In processes requiring ultra pure water with very low concentrations of dissolved oxygen, catalytic deoxygenation has been the preferred method since it adds no foreign or unwanted constituents to the water. Very basically, the reaction to remove oxygen is

(4) 0₂ + 2H₂→2H₂0

Stoichiometry tells us that one mo! of elemental oxygen requires 2 mols of elemental hydrogen so that on a weight basis, 1 weight unit of hydrogen converts 8 weight units of oxygen into water.

While the reaction is simply described, the nature of the reaction requires a catalyst to dxive the reaction to completion in reasonable lengths of time. It has been found that a weak base anion ion exchange resin when doped with palladium metal is an excellent catalyst for reaction described in equation 4 above. . Palladium doped weak base anionic ion exchange resins act as catalysts for deoxygenation with a variety of reducing agents. For hydrazine, the reaction can be described as:

(5) 0₂ + N₂H₄→2H₂0 + N.

Formic acid can also be used with palladium doped weak base anionic ion exchange resin catalysts.

(5) 0₂ + 2HCOOH→ 2H₂ + C0₂

The use of palladium doped weak base anionic ion exchange resins for deoxygenation of high purity liquids has been practiced for decades. This process in clean liquid service such as production of ultra pure water for the electronics industry is not novel. It has been found that this catalytic deoxygenation process can be applied to ISL remediation water. In addition to improving purity of the deoxygenated product, catalytic deoxygenation can reduce operating cost by as much as 75% over the physical and chemical processes described above.

The cost savings of applying palladium doped WBA IEX to the salt and solid laden residual lixiviant from in-situ uranium mining contribute to the attractiveness of the present invention.

As previously stated, catalytic deoxygenation has been used for decades in clean aqueous fluids. The concern has been the effect of high ionic strength in salt solutions on the stability of the palladium attached to the catalyst resin matrix. The water used for down hole injection on off shore gas and oil wells is normally salt water. The corrosive effects of oxygen in salt water under the high temperatures and pressures at the bottom of oil and gas wells are well established. To prevent oxygen induced corrosion, dissolved oxygen is reacted with hydrogen over the palladium doped weak base anionic ion exchange resin. (CANTU, LA.A., ET.AL. TIELD EVALUATION OF CATALYSTl^'C DEOXYGENATION PROCESS FOR OXYGEN SCAVENGING IN OILFIELD WATERS", SPE PRODUCTION ENGINEERING, NOVEMBER 1988, PP. 619 - 624),(GRAN, H.G., ET.AL., "A WEIGHT AND SPACE SAVING SEAWATER INJECTION SYSTEM DESIGN", SPE PRODUCTION ENGINEERING, FEBRUARY 1990, PP. 83 - 84)

Typical wastewater from in-situ uranium mining can be described with the contaminant concentrations shown in Table I. With TDS (total dissolved solids) 3750 ppm(wt), the specified wastewater has a little more than one tenth of the salt concentration of typical oceanic salt water at 35,000 ppm(wt).

Also show in Table 1 is the amount of soluble uranium in the waste water. It is the soluble uranium in the water that could migrate out and potentially contaminate drinking water supplies. While the elimination of dissolved oxygen will stop the oxidation of insoluble uranium (IV) to the soluble form, uranium (VI), it does nothing to reverse the process and stabilize the underground water. To convert uranium (VI) back to uranium (IV), it must be reacted with the proper reagent that will complete the required redox reaction.

Just as oxygen is a very good reagent in oxidation reactions, hydrogen will perform the reverse of the oxidation reaction by giving up its electrons in a reduction reaction. In a basic aqueous environment, activated hydrogen will reduce uranium (VI) to uranium (VI) by

(5) Na₄U0₂ (C0₃ X+ H₂→ U0₂ + 2NaHC0₃ + Na₂ CO

Therefore, just as hydrogen will reduce oxygen from a 0 electrochemical state to a -2 electrochemical state, it will also reduce uranium from a +6 electrochemical state to a +4 electrochemical state. As stated previously, removal of dissolved oxygen from an aqueous stream at moderate temperature and pressure is best facilitated with the use of a Nobel metal catalyst. Of particular interest is the use of a palladium doped ion exchange resin. When the palladium doped ion exchanger is used as the catalyst, we can re-write equation 4 to be

(4a) 0₂ + 2H₂—_Ή→2Η₂0

This equation indicates the use of a catalyst to increase the kinetics of the reaction.

Three examples of this type of catalyst are produced by Lanxess and sold as Lewatit K3433, Lewatit K6333 and Lewatit K7333. Minor imposed variations in physical properties of these three catalysts allow them to have optimal performance in different deoxygenation processes.

These ion exchangers are based upon a polymeric backbone of polystyrene that has been modified for strength by cross polymerization with divinyl benzene. The catalyst has a spherical structure with diameters optimally ranging from 0.4 millimeters to 1.25 millimeters. The size distribution may range from a broad Gaussian distribution to a narrow Gaussian distribution. Those knowledgeable in the art would understand that the uniformity coefficient as described by Dorfner (DORFNER, KONRAD, "ION EXCHANGERS", WALTER DE GUYTER & CO., 1990, PP 311 - 313) can range from 1.05 to 2.0 as required by the process hydraulics. The polymeric based catalysts may have a gel structure or a macroporous structure (Figure 3) depending upon the mass transport needs required by the nature of bulk aqueous fluid. Again, those knowledgeable in the art are familiar with these terms. Further information can be obtained from Dorfner (DORFNER, KONRAD, "ION EXCHANGERS", WALTER DE GUYTER & CO., 1990, PP 320 - 323).

The process for adding elemental palladium to both cationic and anionic ion exchange resins has been well established as well as the use of those catalysts for deoxygenation of water. (US 7,851,406) (TECHNICAL INFORMATION, "LEWATIT® CATALYTIC REMOVAL OF DISSOLVED OXYGEN FROM WATER", BAYER CHEMICALS, CATALYSTS AND CHEMICALS PROCESSING, EDITION 06 2002) As described in (TECHNICAL INFORMATION, "LEWATIT® CATALYTIC REMOVAL OF DISSOLVED OXYGEN FROM WATER", BAYER CHEMICALS, CATALYSTS AND CHEMICALS PROCESSING, EDITION 06 2002), the Lewatit® technical information brochure, this process has been practiced industrially since at least 1979. Concern has been getting the correct addition of hydrogen to the process to fully deoxygenate the process water without adding excess that could result in a hazardous situation.

In the present invention, excess hydrogen addition is desired so that hydrogen is available to reduce water soluble UO₃ to insoluble U0₂ which reverses the reactions described in equations (1) and (2) to give us equation 5a when palladium doped ion exchange resin is used as a catalyst

(5a) Na_AU0₂(C0₃ \ + H₂—_FS→U0₂ + 2N HC0₃ + Na₂CO^

Table 1

Typical Wastewater from In-situ Uranium Mining

Typical

Water

Quality ppm

Alkalinity 900

Ammonium 0.250

Arsenic 0.025

Barium 0.100

Bicarbonate 1100

Boron 1.250

Cadmium 0.010

Calcium 90

Carbonate 0.000

Chloride 600

Chromium 0.050

Copper 0.050

Fluoride 0.500

Iron 0.075

Lead 0.050

Magnesium 25

Manganese 0.075

Mercury 0.001

Molybdenum 0.500

Nickel 0.075

Nitrate 1.000

Nitrite 0.000 pH 7.500

Potassium 30

Radium-226 (pCi/L) 800

Selenium 0.125

Silica 0.000

Sodium 1100

Specific Cond. (uS/cm) 5750

Sulfate 1150

TDS 3750

Uranium 12.500

Vanadium 1.000

Zinc 0.050

Claims

CLAIMS What, we claim:

1. A process for remediating sites used for in-situ leaching for uranium recovery, comprising: providing an ion exchange resin catalyst doped with palladium, hydrogen gas, spent leach water from in-situ uranium leaching, pressure vessels for contacting said water, hydrogen and catalyst, and auxiliary equipment and controls for operation of said process; catalytically converting dissolved oxygen into water in said spent leach water stream and

electrochemically reducing water-soluble hexavalent uranium to water-insoluble tetravalent uranium.