(12) United States Patent (10) Patent No.: US 8,961,911 B2 Dahal (45) Date of Patent: Feb

USOO896.1911 B2

(12) United States Patent (10) Patent No.: US 8,961,911 B2 Dahal (45) Date of Patent: Feb. 24, 2015

(54) SELECTIVE LEACH RECOVERY OF ZINC 5,523,066 A 6/1996 Geisler et al. FROMA COMPOSITE SULPHIDE ORE g R: 1358. R et al DEPOSIT, TAILINGS, CRUSHED ORE OR W - oyes et al. MINE SLUDGE FOREIGN PATENT DOCUMENTS (75) Inventor: Madhav Dahal, Toronto (CA) CN 1414121 A 4/2003 CN 1456692. A 11, 2003 (73) Assignee: Yava Technologies Inc., Toronto, RU 2006 141445 5, 2008 Ontario (CA) WO WO 98.36102 8, 1998 WO WOO3,O23.077 3, 2003 (*) Notice: Subject to any disclaimer, the term of this OTHER PUBLICATIONS patent is extended or adjusted under 35 U.S.C. 154(b) by 4 days. Translation of RU2352652, Apr. 2009.* Translation of CN 1456692, Nov. 2003.* (21)21) Appl.App No.: 13A876,9019 Translation of CN 1414121 Apr.p 2003.* International Search Report of PCT/CA2011/001094, date of mail (22) PCT Filed: Sep. 30, 2011 ing Jan. 6, 2012. International Preliminary Report on Patentability of PCT/CA2011/ (86). PCT No.: PCT/CA2O11AOO1094 001094, dated Apr. 2, 2013, with Written Opinion of the International Searching Authority, dated Dec. 8, 2011. S371 (c)(1), Chinese Office Action dated Apr. 14, 2014 in Chinese Application (2), (4) Date: Apr. 22, 2013 No. 201180057747.5 with English translation. Panamanian Examination Report datedMar. 18, 2014 in Panamanian (87) PCT Pub. No.: WO2012/040829 Registration No. 89635-01. PCT Pub. Date: Apr. 5, 2012 * cited by examiner (65) Prior Publication Data Primary Examiner — Steven Bos US 2013/0216456A1 Aug. 22, 2013 (74) Attorney, Agent, or Firm — Collard & Roe, P.C. Related U.S. Application Data (57) ABSTRACT (60) Provisional application No. 61/404.244, filed on Sep. Zinc and lead are usually concomitantly present in Zn-Pb 30, 2010. ores and tailings. A novel non-polluting hydrometallurgical process for selectively leaching and recovering Zinc (Zn) (51) Int. Cl. from a composite lead (Pb) and Zinc sulphide containing COIG 9/00 (2006.01) mineral, crushed untreated rock or unconsolidated mineral C22B 9/02 (2006.01) particles, mill tailings and/or agglomerated or unagglomer C22B3/00 (2006.01) ated Sulphidic Zinc containing waste material without neces (52) U.S. Cl. sitating Smelting and refining operation has been developed. CPC ...... C22B 19/02 (2013.01); C22B 19/24 A combination of selected oxidant and alkali metal hydroxide (2013.01) has been found effective. A leachant consisting of e.g. a USPC ...... 423/101; 42.3/102:42.3/105. 42.3/100 mixture of sodium hydroxide (NaOH) and Sodium hypochlo (58) Field of Classification se arch s s rite (NaOCl) is employed to selectively dissolve zinc sulphide None at high pH at standard temperature and pressure (STP). The kinetics of leaching along with the effect of varying concen S ee appl1cauonlication fileIlle fIor completelet searcnh historv.n1Story tration (preferably of sodium hydroxide and sodium (56) References Cited hypochlorite) were systematically investigated. Feedore con taining diverse set of minerals e.g. Sulphides and carbonates U.S. PATENT DOCUMENTS can also be conveniently treated to selectively and almost quantitatively recover Zinc as high purity Zinc carbonate. This 3,515,510 A * 6/1970 Satterwhite et al...... 423.109 3,927,170 A 12/1975 Dixon et al. technology can be employed either in-situ or ex-situ based on 4,153,522 A * 5/1979 Arbiter et al...... 205,580 the amenability of a particular type of mineral depositor feed 4,272,341 A 6, 1981 Lamb O. 4.331,635 A * 5/1982 Arbiter et al...... 423.33 4,500,398 A 2/1985 Cole, Jr. et al. 10 Claims, 3 Drawing Sheets U.S. Patent Feb. 24, 2015 Sheet 1 of 3 US 8,961,911 B2

uZ()d??S U.S. Patent Feb. 24, 2015 Sheet 2 of 3 US 8,961,911 B2

1200 ------1000 ------u-uu------...... ------

Time (h)

-- 0.24MNaOCl; 0.675M NaOH -- 0.24MNaOC; 1.35M NaOH 0.48M NaOH; 0.675MNaOH -- a-- 0.48MNaOH: 1.35M NaOH

15 %

10%

5% -- wwww.uuv.

0% - 70 80 Time (h)

-0- 0.24MNaOCl; 0.675M NaOH -- 0.24MNaOCl, 1.3MNaOH 0.48MNaOCl; 0.675M NaOH.-- 0.48MNaOCl; 1.3MNaOH U.S. Patent Feb. 24, 2015 Sheet 3 of 3 US 8,961,911 B2

FIG. 4

100% -

90% 80%

70% 60% 50% | 40% 30% 20% 10% -

O% - alo O 2O 40 60 8O

Time (h)

-0-0.24MNaOCl; 0.675M NaOH -- 0.24MNaOCl, 1.35M NaOH -A-0.48MNaOCl; 0.675M NaOH --0.48MNaOCl, 1.35M NaOH US 8,961,911 B2 1. 2 SELECTIVE LEACH RECOVERY OF ZINC Turner in U.S. Pat. No. 6,726,828, describe use of in-situ FROMA COMPOSITE SULPHIDE ORE leach mining utilizing a mixture of acetic acid and hydrogen DEPOSIT, TAILINGS, CRUSHED ORE OR peroxide (for sulphide oxidation) to recover Ca, Mn, Pb and MINE SLUDGE Zn as a combined leachate from a permeable geological host. Both methods employ hydrogen peroxide as an oxidant. The CROSS REFERENCE TO RELATED decomposition of hydrogen peroxide with time and its effect APPLICATIONS on the overall recovery process is left unexplained. U.S. Pat. No. 4,500,398 uses fluosilicic acid with an oxidant to dissolve This application is the National Stage of PCT/CA2011/ sulphides. Neither of these methods suggests selective leach 001094 filed on Sep. 30, 2011, which claims priority under 35 10 ing of Zinc from composite lead-zinc Sulphidic minerals pro U.S.C. S 119(e) of U.S. Provisional Application No. 61/404, posed herein. 244 filed on Sep. 30, 2010, the disclosure of which is incor porated by reference. The international application under SUMMARY OF THE INVENTION PCT article 21(2) was published in English. 15 A new hydrometallurgical method has been found for BACKGROUND OF THE INVENTION selective dissolution of Zinc from composite Zinc sulphidic minerals. Lead and Zinc Sulphides generally undergo similar oxida The invention comprises a process for selective leaching of tion-reduction reactions. As a result, there is no known Zinc from mixtures and ores containing Zinc Sulphide, com method to leach and recover zinc selectively from composite 20 prising: lead-zinc sulphidic minerals. This invention deals with a a. contacting the mixture or ore with an aqueous leachant selective leaching and recovery of Zinc from composite Zinc comprising: 1) an oxidant selected to oxidize the Sulphur and usually lead-bearing Sulphides, which are either in the present only to elemental Sulphur, and 2) alkali metal form of complex Zinc and lead metal containing Sulphidic hydroxide in amounts sufficient to form soluble alkali minerals, or in the form of zinc Sulphide concentrates, in-situ- 25 metal Zincate; or ex-situ in an economic and environmentally friendly man b. extending the contact time between leachant and Solids . to give the desired zinc recovery and selectivity in the Zinc is the fourth most common metal in use, trailing only leachate while maintaining operative reagent concentra iron, aluminium, and copper. It is normally found in associa tions; tion with other base metals such as copper and lead in natu- 30 c. separating the desired leachate from the residual Solids; rally occurring ores. Zinc has a low affinity for oxides and and prefers to bond with sulphides. Sphalerite, which is a form of d. recovering zinc from the leachate. Zinc sulphide, is the most heavily mined zinc-containing ore. The oxidant may be selected from the group consisting of The major uses of Zinc are anti-corrosion coatings on Steel an oxygen-containing gas, a water-soluble peroxide, a water (galvanizing), precision components (die casting), construc- 35 soluble perchlorate and a water-soluble hypochlorite. tion material, brass, dry batteries, pharmaceuticals and cos Preferably the oxidant is a hypochlorite in a concentration metics and micronutrient for humans, animals and plants. The sufficient to oxidize all of the sulphides present. oxide is used in the manufacture of paints, rubber products, When the starting solids also contain lead sulphide, the floor coverings, plastics, printing inks, soap, textiles, electri resulting leachate is substantially free of lead after an cal equipment, and other products. 40 extended contact time. Conventional extractive metallurgical process generally The desired oxidation potential of the leachant for steps a) involves pyrometallurgical methods for recovering Zinc Val and b) is maintained by reagent addition. The desired alkali ues from Zinc Sulphides. Known recovery process mostly metal hydroxide content of the leachant is maintained involves grinding the ore, froth flotation (which selectively throughout the leaching steps a) and b). The contact time in separates minerals from gangue by taking advantage of dif- 45 steps a) and b) is extended for up to about 24 hours to attain ferences in hydrophobicity) to get an ore concentrate, roast desired recovery and selectivity. ing and reduction with carbon or electrowinning. However, The invention includes an aqueous leachant composition Such treatment often entails expensive mining and beneficia selected to solubilize Zinc selectively from Zinc sulphide tion process steps to concentrate the Sulphides. In addition, containing Sulphidic minerals and mixtures, comprising: the production of Zinc employing the known technology from 50 1) an oxidant selected to oxidize the sulphur from the Sulphidic Zinc ores produces large amounts of Sulfur dioxide, Sulphides only to the elemental Sulphur stage; and carbon dioxide and cadmium vapor. Smelter slag and other 2) an alkali metal hydroxide selected to form soluble alkali residues of process also contain significant amounts of heavy metal Zincates from Zinc Sulphide oxidation products. metals. The dumps of the past mining operations leach sig In a preferred aspect the composite Sulphides are treated nificant amounts of Zinc and cadmium. Soils contaminated 55 with a mixture of sodium hydroxide and sodium hypochlorite with Zinc through the mining of zinc-containing ores, refin at ambient temperature and pressure. Sodium hypochlorite is ing, or where zinc-containing sludge is used as fertilizer, can used as an oxidant to oxidize Sulphide in the composite min contain several grams of Zinc per kilogram of dry soil. Levels eral to elemental sulphur. Zinc oxide thus formed reacts with of zinc in excess of 500 ppm in soil are deemed to interfere sodium hydroxide to form soluble sodium zincate which is with the ability of plants to absorb other essential metals, such 60 Subsequently treated to recover Zinc as high purity Zinc car as iron and manganese. Further, strict adherence to environ bonate, Zinc carbonate can be easily converted to other Zinc mental regulations governing mining operations may Sub products based on end-user requirements. stantially increase the cost of recovering Zinc from its ores by In another embodiment of the invention zinc sulphide con conventional processes. taining unconsolidated minerals, including discrete blocks of A patent search revealed only approaches to simulta- 65 rocks and agglomerated ore particles and concentrate, neously leach both lead and Zinc from composite lead-zinc agglomerated and unagglomerated Zinc sulphide bearing mill sulphidic minerals. Geisler in U.S. Pat. No. 5,523,066 and tailings of mineral beneficiation and similar Zinc sulphide US 8,961,911 B2 3 4 containing by-products and waste products of recycling pro ate. Sodium hydroxide (one of the most common laboratory cesses, are leached ex-situ, at ambient temperature and pres reagents) combined with sodium hypochlorite (commonly Sure, with a solution containing Sodium hydroxide and referred to as bleach) ensures that the reagents utilized in the Sodium hypochlorite. The pregnant leach Solution is Subse leaching process are not likely to damage the environment. quently removed and is treated for Zinc recovery. The leaching process is conducted at ambient temperature and pressure. BRIEF DESCRIPTION OF THE DRAWINGS In one preferred embodiment, at a concentration e.g. of about 0.48M sodium hypochlorite and e.g. about 1.35M In the drawings, which form part of this application: sodium hydroxide, about 96% of zinc was extracted in less FIG. 1 is a flowsheet of the process of the invention. 10 than 24 hours while lead recovery was less than about 1%. FIG. 2 is a graph showing cumulative lead concentration in Zinc leaching kinetics was observed to be the exact opposite solution at various NaOH and NaOCl concentrations; of lead leaching kinetics. While lead recovery percentage FIG.3 is a graph showing NaOH and NaOCl concentration rapidly declined from an initial about 15-25% extraction, influence on lead extraction; and largely attributable to the precipitation of lead as lead dioxide FIG. 4 is a graph showing NaOH and NaOCl concentration 15 due to over-oxidation, Zinc recovery percentage rapidly influence on Zinc extraction. increased initially and formed a plateau. Zinc is recovered from the Solution as Zinc carbonate and Sodium chloride DESCRIPTION OF PREFERRED dissolved in Solution is electrolyzed to regenerate the original EMBODIMENTS leachant forming a closed-loop process. The recovery of metals from their sulphides by hydromet FIG. 1 is a flowsheet of the process of recovery of zinc as allurgical methods usually necessitates the oxidation of the Zinc carbonate/zinc metal from the leachate obtained by sulphide ion in the metal sulphide to render the metal soluble leaching a composite lead-zinc sulfide mineral with a and hence recoverable from the solution. It has been found leachant consisting of a mixture of Sodium hypochlorite and that for best results the sulphide in the sulphidic minerals is Sodium hydroxide. 25 oxidized only to elemental Sulphur, hence the oxidation A leachant consisting of a mixture of sodium hydroxide potential of the oxidant in the leach Solution is adjusted Such and sodium hypochlorite is prepared by diluting concentrated that it is insufficient to oxidize the sulphide to the hexavalent reagent grade solutions to a pre-determined concentration state. The oxidation potential of a reagent is understood to level and mixing them thoroughly in a stirred tank reactor. A mean the power of the reagent to remove electrons and it may composite lead-zinc sulfide mineral is then treated with the 30 be expressed quantitatively in millivolts. In the present pro leachant thus prepared for the dissolution of sulfides by oxi cess for leaching Zinc from Zinc Sulphidic minerals by a dative dissolution process. The leach liquor containing the mixture of sodium hydroxide and sodium hypochlorite, the dissolved metal ions is collected in a pregnant leach Solution oxidant (sodium hypochlorite) could be potentially replaced (PLS) holding tank. Any lead present in the leachate is sepa by oxygen or air, making the process even more economic. rated and recovered as lead metal employing cementation, a 35 Other alkali metals e.g. K could replace sodium. Selective well known art in the industry. Carbon dioxide gas is bubbled dissolution of Zinc Sulphide from composite zinc-lead Sul through the lead depleted leachate to precipitate Zinc as Solid phidic minerals is largely attributed to over-oxidation of lead Zinc carbonate, which is separated by Solid-liquid filtration. leading to reprecipitation of lead as lead dioxide during the Zinc carbonate thus recovered is dissolved is sulfuric acid to leaching process attributable to the following reactions: produce Zinc sulfate solution for electrolytic recovery of zinc 40 as Zinc metal. The lead and Zinc depleted leachate is passed through an electrochemical cell to regenerate sodium hypochlorite. The regenerated solution mixture containing sodium hypochlorite The chemistry involved in the alkaline leaching process is and Sodium carbonate is treated with quick lime or calcium 45 as follows: oxide to precipitate calcium carbonate and regenerate sodium 1. Chlorine and sodium hydroxide are produced by elec hydroxide. The precipitated calcium carbonate is separated trolysis of aqueous Sodium chloride solution. by solid-liquid filtration. The filtrate consisting of a mixture of sodium hypochlorite and sodium hydroxide is recycled for further leaching. Calcium carbonate is roasted to produce 50 2. Sodium hypochlorite is produced by mixing chlorine carbon dioxide gas and calcium oxide. Carbon dioxide gas is with sodium hydroxide. recycled to precipitate Zinc carbonate and calcium oxide is recycled to regenerate Sodium hydroxide solution. The over all process runs as a closed-loop operation. 3. Sodium hypochlorite reacts with zinc sulphide in pres In one aspect of the present process for Solubilizing Zinc 55 ence of sodium hydroxide to produce soluble sodium from composite zinc-Sulphidic minerals in the ore body, Zincate, sodium chloride and elemental Sulphur. crushed ore or tailings, a solution consisting of a mixture of Sodium hydroxide and sodium hypochlorite is used. In one of the preferred embodiments of the present invention the sul 4. Soluble sodium zincate produced in step 3 is treated with phide bearing minerals in the ore are brought into contact with 60 carbon dioxide gas to precipitate insoluble Zinc carbon a mixture of Sodium hydroxide and sodium hypochlorite at ate. high pH. The leach solution reacts with the sulphidic minerals to attain the highest metal ion concentration to render the leaching process economical as determined by the kinetics of 5. Sodium hydroxide is regenerated by treating Sodium the process. The pregnant Solution containing the dissolved 65 carbonate produced in step 4 with quicklime. value metals, in particular solubilized Zinc, are recovered from the leach solution by precipitating Zinc as Zinc carbon US 8,961,911 B2 5 6 6. Calcium carbonate produced in step 5 is calcined to TABLE 1 regenerate quicklime and carbon dioxide gas, which are recycled. Experimental results for leaching composite lead-zinc sulfide mineral Time Volume 96 Lead Extraction 96 Zinc Extraction (h) (ml) % % 7. Pure zinc metal is produced by the electrolysis of zinc a) 1 450 11% SO% Sulphate solution produced by dissolving Zinc carbonate 0.24MNaOCl. 2 442 9% S8% O.67SMNaOH 4 434 6% 66% precipitate from Step 4 in Sulphuric acid. 24 426 190 76% 10 48 418 2% 74% S1 460 190 74% 72 452 O% 75% b) 1 450 25% 46% 0.24MNaOCl. 2 442 23% 57% 1.35M NaOH 4 434 21% 69% A bleed solution is intermittently treated to remove the impu 15 24 426 14% 84% rities built up during the leaching process. 48 418 79% 83% S1 460 4% 85% The present invention has the additional advantage that it 72 452 O% 86% does not entail preconcentration of the minerals, which may c) 1 450 59% 55% require costly mining expenditures and equipment. The pro 0.48MNaOCl. 2 442 59% 64% O.67SMNaOH 4 434 4% 729% cess does not create acid drainage problems and uses rela 24 426 190 81% tively environmentally benign reagents. 48 418 190 79% 72 410 190 74% d) 1 450 20% 61% Example 1 0.48MNaOCl. 2 442 1796 71.9% 1.35M NaOH 4 434 15% 80% 25 24 426 190 96% 50 g of crushed ore was placed in a bottle with 450 ml 48 418 190 95% lixiviant. The lixiviant was prepared by mixing 300 ml con 72 410 O% 89% Sumer grade sodium hypochlorite (NaOCl) with 150 ml deionized water and 24.3 g sodium hydroxide (NaOH). The FIG. 2 shows the quantity of lead that remains dissolved in target concentrations prior to testing were 1.35M NaOH and 30 0.6M NaOCl. The mixture was continuously stirred with a the leachate after leaching a composite lead-zinc sulfide min magnetic stirrer. 20 ml samples were collected at fixed inter eral employing a leachant consisting of a mixture of sodium val of time and quantitatively analyzed for both lead and Zinc hydroxide and sodium hypochlorite. The effect of variable concentration. Approximately 96% of zinc was recovered in concentrations of Sodium hypochlorite at various sodium less than 24 hours. Lead concentration in the Solution at the hydroxide concentrations clearly indicates that there is a end of 24 hours period of the experiment was found to be less 35 rapid decrease in the quantity of dissolved lead in the leachate than 1%. with time. FIG. 3 shows the kinetic efficiency of lead extraction on Example 2 leaching a mixture of composite lead-zinc sulfide mineral 40 employing a leachant consisting of a mixture of Sodium hydroxide and sodium hypochlorite. The effect of variable Column test was conducted to mimic in-situ leaching. Approximately 120g crushed ore, containing composite lead concentrations of Sodium hypochlorite at various sodium and Zinc sulphidic minerals was lightly ground with a mortar/ hydroxide concentrations again indicates that there is a rapid pestle and packed in a 1.27 cm-ID (internal diameter)x51 decrease in the efficiency of lead extraction with time. cm-L clear vinyl tube. Small plugs of glass wool were placed 45 FIG. 4 shows the kinetic efficiency of zinc extraction on on the ends of the tubing, acting as particulate filters as the leaching a mixture of composite lead-zinc sulfide mineral liquid goes through the column, Tapping the sides of the employing a leachant consisting of a mixture of Sodium column ensured uniform packing. Prior to leaching, N. hydroxide and sodium hypochlorite. In direct contrast to lead sparged deionized water was pumped through the column to extraction efficiency, the effect of variable concentrations of 50 Sodium hypochlorite at various sodium hydroxide concentra remove any entrapped air. The deionized water was left in the tions clearly indicates a rapid and a highly efficient recovery sealed column overnight. of zinc extraction with time. The lixiviant (0.675M NaOH and 0.48M NaOCl) was Although the present invention has been described with pumped upward through the column, at relatively constant reference to the preferred embodiments, it is to be understood flow rate using a peristaltic pump. The effluent was collected 55 that modifications and variations may be resorted to without in a separatory funnel. 10-15 ml aqueous samples were col departing from the spirit and scope of the invention, as those lected at the exit of the column at pre-set time intervals and skilled in the art readily understand. Such modifications and quantitatively analyzed for lead and Zinc concentration. The variations are considered to be within the purview and scope target flow rate was 1 ml/min, translating into approx 20 of the invention and the appended claims. minutes residence time in the column. The actual average 60 flow rate throughout the 22.5 hours testing period was 1.05 I claim: ml/min. While approximately 81% of zinc was recovered, 1. A process for selective leaching of zinc from a mixture or only about 1% lead was extracted. ore containing Zinc sulfide, comprising: Detailed kinetic leaching tests were performed at various a. contacting the mixture or ore with an aqueous leachant to Sodium hydroxide and sodium hypochlorite concentrations. 65 form a resulting leachate and residual Solids from the Table 1 summarizes the experimental results illustrated in mixture or ore, said leachant comprising: 1) an oxidant FIGS. 2, 3 and 4. with an oxidation potential to oxidize the sulfur present US 8,961,911 B2 7 8 only to elemental sulfur, and 2) alkali metal hydroxide in 7. The process of claim 1, wherein the amount of alkali an amount Sufficient to form soluble alkali metal Zincate; metal hydroxide in the leachant is maintained throughout the b. extending the contact time between leachant and solids leaching steps a) and b). to reach a selected zinc recovery and selectivity in the 8. The process of claim 1, wherein the contact time in steps a) and b) is extended for up to about 24 hours to attain the leachate while maintaining operative reagent concentra selected Zinc recovery and selectivity. tions; 9. The process of claim 1, in which zinc in step d) is c. separating desired resulting leachate from the residual recovered by precipitation as Zinc carbonate. Solids; and 10. A process for selective leaching of zinc from a mixture d. recovering Zinc from the resulting leachate, or ore containing Zinc sulfide, comprising: wherein the oxidant is a water-soluble perchlorate or a 10 a. contacting the mixture or ore with an aqueous leachant to water-soluble hypochlorite. form a resulting leachate and residual Solids from the 2. The process of claim 1, wherein the oxidant is sodium mixture or ore, said leachant comprising: 1) an oxidant hypochlorite. with an oxidation potential to oxidize the sulfur present only to elemental sulfur, and 2) alkali metal hydroxide in 3. The process of claim 1, wherein the oxidant is a water 15 an amount Sufficient to form soluble alkali metal Zincate; soluble hypochlorite in a concentration sufficient to oxidize b. extending the contact time between leachant and Solids all of the sulfides present. to reach a selected zinc recovery and selectivity in the 4. The process of claim 1, wherein the leachant contains leachate while maintaining operative reagent concentra Sodium hypochlorite and sodium hydroxide. tions; 5. The process of claim 1, wherein the mixture or ore also c. separating the resulting leachate from the residual Solids; contains lead Sulfide, and the resulting leachate is Substan and tially free of lead after an extended contact time. d. recovering Zinc from the resulting leachate, 6. The process of claim 1, wherein the oxidation potential wherein the oxidant is sodium hypochlorite and the alkali of the leachant for steps a) and b) is maintained by reagent metal hydroxide is sodium hydroxide. addition. k k k k k UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. : 8,961,911 B2 Page 1 of 1 APPLICATIONNO. : 13/876901 DATED : February 24, 2015 INVENTOR(S) : Dahal It is certified that error appears in the above-identified patent and that said Letters Patent is hereby corrected as shown below:

In the Claims, Column 7, line 7 (Line 1 of paragraph c. in Claim 1), please change “desired to correctly read:

--the--.

Signed and Sealed this Second Day of June, 2015 74-4-04- 2% 4 Michelle K. Lee Director of the United States Patent and Trademark Office