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DOUGLAS M. PROVOW and RAY W. FISHER Ames Laboratory of the U. S. Atomic Energy Commission, Institute for Atomic Research, Iowa State University, Ames, Iowa
Chemical Processing of Yttrium Scrap Low cost process uses readily available chemical reagents and equipment to reclaim high cost rare-metal
T m yttrium metal at the Ames Laboratory resulted in the PRODUCTION
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OF
accumulation of large quantities of scrap. This was scrap from metal purified by an ion exchange process and was in the form of small pieces, turnings, and saw filings. Due to the oxide scale, dirt, and other impurities, no method had been developed for remelting this scrap to a metal of satisfactory purity. A chemical process has been developed for the economical purification and conversion of this scrap to a material that could be used in the production of very pure metal with special emphasis given to the removal of impurities which would otherwise continually build u p in the metal production. The ion exchange method of Powell and Spedding (5) which was used with remarkable success to separate adjacent rare earths was considered for the purification of this scrap. Since no rare earths were present as impurities in the scrap, the ion exchange method was considerably more expensive than the chemical methods for the relatively easier task of separating the nonrareearth impurities from yttrium. Some of this scrap, especially the finely divided metal, constituted a fire hazard as the metal burned freely in the air. These scrap materials were burned as they were produced to reduce this fire hazard.
Procedure
Crude Yttrium Oxide. The scrap materials, turnings, saw filings, and oxide contained a total of approximately 1% impurities. This scrap could have been treated directly in acid but because of the hazard caused by the evolution
of hydrogen it was burned on an open fireplace. A quarter pound of turnings was placed on firebrick and ignited. Small quantities of scrap were added to sustain the reaction. This fire produced a clinker containing some unreacted metal. The clinker was broken and placed in silica trays where it was ignited a t 800' C. to the oxide in an electric muffle. This crude oxide was 50% minus 80 mesh. Dissolution. One hundred pounds of crude yttrium oxide were added to a SOTOnitric acid solution at the rate of 4 pounds per minute until the heat of reaction caused the solution to boil. An optimum ratio of 3.26 pounds of %)Yo nitric acid was used per pound of crude oxide which was approximately a stoichiometric amount. This ratio is
important as excess acid necessitates the addition of more crude oxide. When the yttrium oxide addition was complete, steam was sparged into the vessel to continue the reaction and was shut o f f after the solution reached pH 4.1. The reaction was complete when the solution reached pH 3.8. At this point the solution was diluted to 1.5 pounds of yttrium oxide per gallon to reduce the viscosity of the solution and to bring the solution above pH 4.1 (usually to 4.6). The precipitation of the zirconium, iron (3),aluminum, and titanium impurities was complete when the solution reached pH 4.1 (6). Impurity Removal. Potassium ferrocyanide, added to the cooled solution in a ratio of 1.5 times the stoichiometric amount of the copper and nickel, pre-
These Items Were Used to Recover the Expensive Scrap Equipment Clarifier for filtering-Alsop Engineering Corp. Reaction vessels-glass-lined tanks with stainless steel stirrers Precipitation and filtration tanks-glass-lined, rubber- or plastic-lined or made from stainless steel
Commercially Available Chemicals and Materials Celite filter aid-Johns-Manville Nitric acid Sulfuric acid Potassium ferrocyanide
Corp. Ammonium hydroxide Oxalic acid Oxalic acid anhydride
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cipitated these impurities. I n this case, about one-fourth pound of potassium ferrocyanide per 100 pounds of yttrium oxide was used. When the impurities had been precipitated, the further addition of ferrocyanide served no useful purpose and began to precipitate the yttrium. After the impurities had been permitted to settle, the solution was decanted and filtered through an Alsop clarifier containing asbestos disks. A diatomaceous earth filter-aid, Celite (Johns-Manville), was used at the rate of a quarter pound per 100-pound batch to aid in clearing some of the solutions. The residue from the reaction tank, which contained as much as 0.3y0 of the original yttrium oxide, was collected from several runs, treated with nitric acid (or sulfuric acid if dissolution was difficult) until the yttrium was in solution. This solution was neutralized with 3y0 ammonium hydroxide. diluted, filtered, precipitated with oxalic acid, and recycled. Oxalate Precipitation. Batches, containing the equivalent of 13 pounds of yttrium oxide diluted to 100 gallons, were precipitated with technical grade oxalic acid and stirred until the acid dissolved. Two pounds of oxalic acid dihydrate were added per pound of oxide [2Oyo more than a stoichiometric amount ( Z ) ] . The clear supernatant was checked for complete precipitation with a solution of oxalic acid, allowed to settle for 3 hours, and decanted. The precipitate was filtered, washed, and ignited in silica trays a t 800’ C. The oxide obtained was 99.5% minus 80 mesh and of satisfactory purity. Tests indicated that less than 2% calcium could be removed by this step where the supernatant was p H 0.46 If the alkaline earths are present in large quantities, the yttrium could be separated by an ammonium hydroxide precipitation (7). The solubility of yttrium was determined on the clear supernatant from the oxalate precipitation. The total yttrium in solution represented 0.4% of the batch. Subsequent washing of the precipitate showed only 0.04% of the batch remained in solution, which was equivalent to the amount calculated from solubility curves ( 4 ) .
Discussion The main features of this recovery process were the low costs and the purity of the oxide produced. The product obtained by one oxalate precipitation was comparable with the best grades of column-run oxide. The chemical costs of a large scale operation was 67 cents per pound of yttrium oxide and could be considerably reduced by recycling the oxalate (7) and nitric acid.
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CL AR IFI CAT1ON
FILTER
DECANT
REACTION TANK
NITRATE
STEAM
DISTILLED WATER OXALIC ACID
DECANT TO ACID RECOVERY PRECIPITATION
1 FILTER TRAYS
In reclaiming pure yttrium from scrap, the principle steps are dissolution with nitric acid, precipitation of the main impurities as hydroxides, and precipitation of yttrium
Over 500 pounds of purified yttrium oxide have been recovered using this process. The oxide can be easily hydrofluorinated and over 300 pounds of oxide have been treated in this way. Modifications have been tried with success. Yttrium oxide containing large quantities of copper impurities were treated with nitric acid and allowed to react until the pH was greater than 4.1. Hydrogen sulfide was introduced and the precipitate was filtered in the usual manner. No copper was detected in the yttrium oxide product. Yttrium sponge has been purified by direct treatment with nitric acid where the excess hydrogen could be handled. In this case an excess of metal was used to ensure that the p H was elevated above 4.1 after which the rest of the process was continued. An alloy of 25y0 magnesium was dissolved successfully with 20y0 nitric acid and processed in the usual manner after the solution reached pH 4.1. The product obtained contained less than 100 p.p.m. magnesium. The reaction was carried out in glasslined tanks but stainless steel could be used as the stirrers employed were of this type of material. The precipitation and filtration steps can be processed in glass-lined, rubber- or plastic-lined, or stainless steel equipment. This method of recovery could be used for the purification of other rare-earth scrap which contains no other rare earths as impurities.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Analysis Shows Yttrium Scrap Can Be Processed to High Purity Oxide P.P.M. Crude Processed Impurity oxide oxide Zirconium Iron Calcium Nickel Silicon Copper Magnesium Lithium Chromium Manganese Aluminum Titanium Potassium Sodium 5
6000 1000 980 390 330 50 30 50
300 50
