Table of Contents
- Precipitation of Vanadium
- Nitrate Recovery
- Use of Low-Grade Nitrate Unsatisfactory
- Storage of Nitrate Solution
- Method of Using Crystallizing Pans
- Nitric Acid Manufacture
- Uranium Refining
- Methods First Used
- Results Obtained by Various Investigators
- Fusion Method
- Use of Oil Furnaces
- Fusion with Sodium Salts
- “Light” and “Dark” Sodium Uranate
- Conversion of Sodium Uranate Into Uranium Oxide
- Effect of Reduction
- Plant Procedure Tried
- Small-Scale Tests of Other Processes
- Tests of Various Crucible Materials
- Construction of Experimental Electric Furnace
- Voltages Used
- Melting Pots Used
- Product Obtained
- Explosions in Electric Furnace
- Cost of Treatment with Electric Furnace
As the sodium uranate requires re-treatment, owing to the fact that it carries vanadium, it is not necessary to wash the cake as completely as might otherwise be required. Most of the sodium uranate carries 7 to 9 per cent V205. It has been found practically impossible to obtain a precipitate that does not carry considerable quantities of vanadium, most of which appears probably as uranium vanadate.
Precipitation of Vanadium
The filtrate from the sodium uranate precipitate contains the vanadium as sodium vanadate. The solution is brought to the boiling point and just neutralized with nitric acid, the boding being continued long enough to eliminate the carbon dioxide. A workman, after a little practice, is able to neutralize the solution and do the rest of the work satisfactorily. A solution of ferrous sulphate, made by dissolving ordinary commercial ferrous sulphate in cold water, is then run from small storage tanks (38, fig. 4 and Pls. V and VII) into the hot solution, agitation being accomplished by means of compressed air. The amount of ferrous sulphate added depends to some extent upon conditions, such as acidity, irrespective of the amount of vanadium present. As a rule, about 75 pounds of ferrous sulphate is required per ton of ore treated. The heating of the solution is stopped before the addition of the ferrous sulphate, for if heating is continued longer a complete precipitation of the vanadium is not obtained.
It is advisable to have the solution just neutral after the addition of the sulphate, and if it is neutral before this addition it will of course be slightly acid afterwards. It is difficult so to gauge the amount of acid added to the vanadium solution that the latter will be exactly neutral after the addition of the ferrous sulphate. In practice it is found convenient to make the solution exactly neutral before the addition of the ferrous sulphate and then to add a few pounds of sodium hydroxide to neutralize the solution once more after the addition of the ferrous sulphate. In this way practically all of the vanadium is precipitated as iron vanadate, only a mere trace going through in the filtrate. The precipitate is probably a mixture of the different vanadates of iron and has a greenish-gray color; it usually contains 32 to 33 per cent of V2O5.
It is not difficult to control the grade of the precipitate obtained. If the solution is slightly alkaline before the addition of the ferrous sulphate, a product carrying as low as 25 per cent V2O5 may be obtained. This is brown, with practically no green tinge. If the solution is faintly acid after the addition of the ferrous sulphate, a product may be obtained that is somewhat yellow and under favorable conditions may carry as much as 42 per cent V2O5. In other words, in a slightly acid solution, with continued boiling, a high-grade product is precipitated, but all of the vanadium is not recovered, some of it going into the filtrate. This contaminates the sodium nitrate and undoubtedly causes losses on evaporating the nitrate solution owing to decomposition.
If carnotite is treated in a beaker with nitric acid, filtered, the filtrate poured into an excess of hot sodium carbonate solution, the uranium precipitated by sodium hydroxide, and the vanadium precipitated by ferrous sulphate the product universally obtained is a dark-brown precipitate carrying less than 25 per cent V2O5. On a large scale oxidation is much more complete, and a much higher grade product can be obtained than in the laboratory. Seemingly this result is due not only to the long time during which the liquids are boiled, but also to the fact that air is being continually passed through
Before the erection of the nitric-acid plant the neutralization of the vanadium solution was accomplished by means of sulphuric acid instead of nitric acid. The change was made for reasons already indicated under the discussion of uranium precipitation.
Before the second plant had been built, the evaporator and the crystallizing pans for the recovery of the sodium nitrate were placed in the angle formed by the first plant and the boiler house, which is now used as a furnace room. After the second plant had been added, the crystallizing pans were removed from their original position to that shown in Plate VI (44). An extra evaporator was also installed to take care of the increased capacity. The present equipment handles all of the nitrate solution from both plants.
As already stated, while the first plant was being operated by itself, the excess sodium carbonate in the uranium and vanadium tanks was neutralized by means of sulphuric acid, consequently the filtrate from the vanadium precipitate consisted of a neutral solution of sodium nitrate and sodium sulphate. It was necessary, therefore, in order to recover a sufficiently high grade of nitrate, to separate a considerable amount of the sulphate from the nitrate by fractional crystallization.
The filtrate from the vanadium precipitate was stored in tanks 11 and 12 (Pls. III and V) before the liquor was run into the evaporating pan. One of these tanks was fitted with a grid made of Byers pipe. Waste steam was run through the grid so that partial evaporation took place in the tank itself before the liquor was run into the evaporating pan, in which it was finally concentrated to a specific gravity of 1.23 to 1,27. As concentration was effected in the pan, more liquor was pumped, from the storage tank. When a sufficient amount of the concentrated liquor had accumulated, it was run into a crystallizing pan (Pl. XI, A). The product obtained from this first crystallization, consisting mostly of sodium sulphate, varied somewhat in composition, the variation depending largely on the temperature of the air during crystallization. Usually, however, the wet salt carried between 3 and 10 per cent of sodium nitrate, the water content varying from 42 to 50 per cent, and was classed as “high-grade sulphate.” As it could not be sold or even given away in Denver, most of it was dumped.
PART 4: RADIUM Extraction & Recovery
The mother liquor from the crystals was pumped back into the evaporator and concentrated to a gravity of 1.35 to 1.38, and allowed to crystallize. The product obtained from this crystallization was classed as “low-grade nitrate,” and as with the first crystallization, the composition varied somewhat, according to the temperature of the air during the crystallization. Most of the wet product carried 30 to 35 per cent sodium nitrate and 20 to 30 per cent water. The liquor from the low-grade nitrate was further concentrated to a gravity of 1.40 to 1.44 and again allowed to crystallize. The material obtained from this crystallization was classed as “high-grade nitrate,” and the wet salt contained 65 to 80 per cent sodium nitrate and between 6 and 10 per cent water. The mother liquor from these crystals was added to the next batch of liquor from the low-grade nitrate.
Use of Low-Grade Nitrate Unsatisfactory
In the making of nitric acid from this recovered nitrate it was necessary to work in the low-grade nitrate in small quantities with the high-grade nitrate or fresh purchased nitrate. Even under these conditions the amount of moisture and sodium sulphate present in the low-grade nitrate made its use unsatisfactory. When the losses were taken into consideration, as well as the cost of the additional sulphuric acid required because of the presence of the large amount of sodium sulphate in the nitrate, it was found not only more satisfactory, but actually cheaper, to use nitric instead of sulphuric acid in the neutralization of the sodium carbonate in the uranium and vanadium tanks. Under these conditions it is not necessary to crystallize out any sodium sulphate, as the amount present is small, coming from the ferrous sulphate and also the sulphuric acid used in the precipitation of the radium barium sulphate. The nitrate obtained under these conditions usually runs from 80 to 85 per cent NaNO3.
Storage of Nitrate Solution
According to the present arrangement, tanks 11 and 12 in the new plant (Pl. VI) are used as storage tanks for the nitrate solution from both plants, as the vanadium precipitate from both plants is filtered through filter press 18 (Pls. VI and VII) in the new plant. Partial evaporation takes place in both tanks, waste steam being used. The liquor is then pumped from the tanks into the evaporators (48, Pl. VI), where evaporation is carried on until a gravity of 1.4 is obtained. At this gravity crystals will usually separate out to some extent in the hot liquor, and on running the liquor into the crystallizing pans crystals are frequently found on the bottom of the evaporator.
The eight crystallizing pans are divided into two groups of four, each group taking care of one day’s run from both plants, so that crystallization takes place over a period of 48 hours. The mother liquor from the first crystals is pumped through a Worthington pump back into the evaporator, this pump being the same as is used to pump the liquor from the reservoir tanks into the evaporators. On the second evaporation the gravity is run to between 1.44 and 1.46, the mother liquor from these crystals being added to the next batch of fresh material.
The sodium nitrate obtained after the removal of the mother liquor is shoveled out onto the draining boards which are built over the crystallizing pans. After thorough drainage the nitrate is removed to the nitrate storage house, situated between the crystallizing pans and the nitric-acid plant.
Method of Using Crystallizing Pans
The actual procedure in using the different crystallizing pans is as follows :
The mother liquor from pans 1, 2, 3, and 4 (Pl III), or in reverse order, is pumped into one of the evaporators as needed. After all of the mother liquor from the four pans has been introduced into the evaporator, the crystals from the previous crystallization are shoveled from the pans onto the draining boards (Pl. XI, A). The liquor in the evaporator is then run into pan 1 for the second crystallization. Sometimes the volume of this liquor is large enough to make the use of pan 2 also necessary. The nitrate solution, which has been more or less concentrated in the storage tanks (11, 12, Pls. VI and VII), meanwhile has been further concentrated in the other evaporator.
When a gravity of 1.4 is reached, the liquor is run into pans 2, 3, and 4 for the first crystallization. The next day the same process is repeated with pans 5, 6, 7, and 8, the mother liquor from the second
crystallization in each instance going back into the fresh material from the plant.
Since both plants have been running full capacity, the average weight of sodium nitrate recovered per day has been close to 4,000 pounds on the 100 per cent basis. This, of course, has varied to some extent, as the recovery has varied also. The average analysis of the nitrate during this period has been as follows:
Nitric Acid Manufacture
A plan of the nitric acid plant is shown in. Plate XII. The sodium nitrate is elevated by means of the elevator c, and is bedded and sampled at b. As the composition of the nitrate varies to some extent, every day, it is necessary to make a daily analysis of the nitrate used. The charge is wheeled in wheelbarrows from b to the stills d. The bleacher and the Hart condensers are shown at e, the towers at f, and the storage pots at g. As the manufacture of nitric acid is more or less standardized, it is not necessary to describe it in detail in this report. The plant has produced an average of 5,250 pounds of acid per day over a period of several months, the whole of the acid averaging 63 per cent in strength.
As already stated under the description of the precipitation of uranium, most of the sodium uranate that is obtained contains 7 to 9 per cent of V2O5 on the dry basis. It has been impossible to precipitate sodium uranate in alkaline solution containing both uranium and vanadium and not precipitate at the same time a considerable amount of vanadium with the uranium. As uranium should be as free from vanadium as possible, the economical refining of the uranium is important.
Methods First Used
The first attempts to remove the vanadium from the sodium uranate were along the line of reprecipitation. The sodium uranate was redissolved in hot dilute sulphuric acid and sodium hydroxide added in sufficient excess to reprecipitate the uranium. It was found that one precipitation carried on in this manner reduced the amount of V2O5 in the sodium uranate by about 50 per cent. 2 second reprecipitation reduced the amount of V2O5 that was left in the product again to about half, so three, and sometimes even four, reprecipitations would be necessary to reduce the V2O5 content below 1 per cent. Such a procedure, of course, would not be commercially feasible. Consequently, it was necessary to find some other methods that would be more economical.
If sodium uranate containing vanadium is heated with somewhat concentrated nitric acid, evaporated almost to dryness and then treated with water, the larger part of the vanadium remains undissolved, but the vanadium carries considerable uranium, and the uranium still retains some of the vanadium. If more dilute acid is used and boiling continued for only a few minutes, practically all of the vanadium can be precipitated, but the precipitate still retains a considerable amount of uranium. Somewhat the same results can be obtained with hydrochloric or sulphuric acid, but with either, the precipitate of vanadium carries down considerable quantities of uranium.
Results Obtained by Various Investigators
Smith and Gibbs have shown that vanadic acid can be removed from heated sodium uranate by means of hydrogen chloride, the vanadium volatilizing. Hillebrand has shown that vanadium also can be partly removed in the same manner from ores. In the case of the sodium uranate, the product left behind is probably a mixture of sodium chloride, uranyl chloride, and sodium uranate. Barker obtained similar results. He also removed vanadium from sodium uranate by mixing the uranate with twice its weight of ammonium chloride and enough water to make a thick paste. On heating, the vanadium may be practically completely volatilized, the amount left in the residue being reduced to as low as 0.5 per cent. The ammonium chloride at the same time converts the uranium present to ammonium uranate, which yields uranium oxide. If the temperature is too high, the amount of oxide obtained is reduced materially, as some of the oxide is converted back to sodium uranate. The best conditions are obtained when the temperature is not raised higher than is necessary to volatilize the vanadium and the ammonium chloride.
Any of these methods might be used for the removal of the vanadium, but on a commercial scale practically all of them involve serious difficulties. The use of nitric, hydrochloric, or sulphuric acid would give excellent results, provided the precipitation between the vanadium and uranium was sharp and complete, but if all the vanadium is to be removed such a separation is practically impossible. Too much uranium goes with the vanadium, and if the loss of uranium is reduced, a considerable amount of vanadium is not separated from the uranium.
The authors have found, however, that the vanadium can be separated from sodium uranate cheaply and efficiently by fusion methods. If sodium uranate is heated with two to two and one-half times its weight of sodium sulphate until the whole mass is fused, and the product is afterwards broken up and leached, practically all of the vanadium goes into the solution and the uranium still remains as insoluble sodium uranate. In this manner the vanadium content of sodium uranate can be reduced from 8 or 10 per cent to less than 0.5 per cent by one treatment, practically all of the vanadium being removed if the product is sufficiently washed.
The main trouble with this method is in finding a satisfactory container for the material during fusion. Steel is slightly attacked by fused sodium sulphate, and cast iron, although standing up much better under the action of the molten material, has too low a melting point to make its use advisable for straight sodium sulphate. Although the sulphate has a melting point much lower than that of cast iron, when considerable quantities of sulphate are used it is difficult to get the center part of the mass melted before the exterior reaches the melting temperature of cast iron. Vitrified brick stands up well, and therefore a small reverberatory furnace could be used. The authors have found it more convenient, however, to add a small quantity of sodium nitrate to the sodium sulphate, in order to lower the melting point of the sulphate. If 20 to 25 per cent of sodium nitrate is added to the sodium sulphate, a melt can be readily obtained in a cast-iron pot.
Use of Oil Furnaces
Oil furnaces with Case burners (Pl XI, B) have been used at the plant of the National Radium Institute, the pots holding 150 pounds of the mixture of sodium nitrate and sodium sulphate. To this melt is slowly added from a hopper 40 to 50 pounds of sodium uranate, with frequent stirring by means of an iron rod flattened at the end and inserted in a wooden handle. Care must be taken that effervescence does not occur too rapidly, as otherwise the pot may boil over.
After the sodium uranate has been completely added, heating must be continued for about one hour longer, in order that all of the material may be acted upon by the melt.
It is run out through a spout at the bottom of the pot into a suitable iron container, broken up, run through a ball mill, either dry or, preferably, with water, and leached with boiling water in a suitable tank. The leaching is done in tank 4 (Pls. III and IV, B) in the first plant, and the product pressed through filter press 33b (Pl. IV), the filtrate running into tank 7 (Pl III), in which the vanadium is precipitated. The sodium uranate thus obtained, provided the fusing has been carried out properly, is practically free from vanadium.
Fusion with Sodium Salts
This procedure fitted in excellently with the main process, as sodium sulphate containing sodium nitrate was one of the by-products in the crystallizing of the sodium nitrate, and therefore purchase of the material was not necessary. Other sodium salts besides sodium sulphate, or a mixture with sodium nitrate, have a similar effect. Fused sodium hydroxide, sodium carbonate, or a mixture of sodium hydroxide and sodium carbonate, removes vanadium from sodium uranate, the sodium uranate being insoluble in a leaching process, but neither the hydroxide nor the carbonate gives as good a result as sodium sulphate. Ordinary salt, however, is just as efficient as sodium sulphate. In addition it has the advantage of not attacking cast steel, so that steel pots may be used and the salt readily melted therein. For this reason salt was substituted for the mixture of sodium sulphate and sodium nitrate at the Radium Institute.
“Light” and “Dark” Sodium Uranate
The sodium uranate that is obtained by this fusion method is off color. In order to obtain the light-yellow material, re-treating would be necessary. If the uranium is present as double sodium uranyl carbonate, the color of the product can be controlled without much difficulty. If acid is added to the solution until neutralization is almost complete, the precipitate that comes down is light yellow, sometimes almost white. If before neutralization is actually completed, sodium hydroxide is added until all of the uranium is precipitated, the color of the precipitate will be light yellow. The same result can be accomplished by adding acid until the solution is slightly acid, and then adding sodium hydroxide until precipitation is just complete. In order to get the dark variety of sodium uranate by direct precipitation, all that is required is to add sodium hydroxide to the solution of sodium uranyl carbonate without previous partial or complete neutralization. It is also advisable not to wash too thoroughly the sodium uranate obtained in this manner. Unless there is no objection to the presence of vanadium the precipitation of part of the vanadium along with the uranium, of course prevents the direct handling of the material in the manner described.
Conversion of Sodium Uranate Into Uranium Oxide
The conversion of sodium uranate into uranium oxide is also of interest. The first work of this kind done by the authors involved precipitation with ammonium hydroxide in order to obtain ammonium uranate, so that on ignition of the uranate U3O8 could be obtained. Boiling the sodium uranate with a strong solution of ammonium chloride was also tried. Some laboratory experiments were carried out on these methods before they were used in the plant, with the following results:
A 5-gram sample of dry sodium uranate was dissolved in 3 c. c. of concentrated sulphuric acid diluted with 25 c. c. of water, and the solution was poured into 25 c. c. of ammonia, the total volume being made up to 100 c. c. Excess of ammonia was expelled by boiling until rather violent bumping occurred. The yellow precipitate was filtered off and washed twice on the filter and ignited. Another sample of the same weight was boiled with 50 grams of ammonium chloride in 50 c. c. of water. The liquid was boiled for half an hour, a drop of ammonia being added from time to time to maintain an alkaline solution. The precipitate was treated as before. The weight of the residue obtained from the first experiment was 3.99 grams, and from the second 3.85 grams.
Each residue was digested with 1 c. c. of sulphuric acid in 25 c. c. of water, the weight of the residue obtained from this treatment being 2.47 grams for the ammonia-treated material and 2.71 for the other. The difference does not represent solely the amount of sodium uranate with the uranium oxide, as under the conditions at least 15 per cent of U3O8 is dissolved by the acid. For the final product, U3O8 is what is desired, and the experiments showed that by the use of a large excess of ammonium chloride a yield of 54 per cent of U3O8 is obtained, and of 50 per cent by digestion in ammonium sulphate in excess.
Further experiments showed that 2 parts by weight of ammonium chloride to 1 part of uranate gave practically as good a yield as when 5 parts of the chloride were used, and the yield was not very much reduced when the amount of chloride was still further decreased so that the ratio was 1.5 to 1.
Effect of Reduction
In the course of the experiments it was observed that in dissolving the sodium uranate in concentrated sulphuric acid and in passing from sodium uranate to the oxide a partial reduction to uranous salts took place. It was therefore thought possible that the efficiency of the conversion might be increased by reducing the sodium uranate before its precipitation as the ammonium salt. The reduction was accomplished by adding to the sulphuric acid a solution of sulphurous acid and boiling to expel the excess of sulphur dioxide.
The uranium was then precipitated by ammonium hydroxide in the ordinary way. The efficiency showed a slight increase, but the difference was within the limits of experimental error.
The results of the work clearly indicated that it is difficult to completely convert sodium uranate into ammonium uranate by one treatment with ammonium chloride or ammonium sulphate, the conversion into ammonium uranate being usually somewhere around 60 per cent. In other words, a mixture of ammonium uranate and sodium uranate is obtained. On ignition the ammonium uranate is, of course converted into oxide, which remains contaminated with the unconverted sodium uranate. The latter can be leached out by dilute sulphuric acid, the preferable strength being 4 or 5 per cent. In this manner a fairly pure oxide can be obtained, which, however, usually contains traces of sodium uranate, iron, silica, and aluminum.
Plant Procedure Tried
The actual procedure which was finally used in experimental work in the plant was as follows:
One hundred pounds of the refined sodium uranate was dissolved in 100 pounds of 66° B. sulphuric acid diluted with 200 pounds of water. The solution took place in an earthenware pot (84, Pl. IV). The sodium uranate dissolved but the majority of the iron oxide, as well as some other impurities, remained insoluble. Most of the iron was derived from the impurities in the sodium sulphate and sodium nitrate used in the fusion method for eliminating the vanadium, as already described. The iron oxide could be removed by filtration through an earthenware filter (35, Pl. IV) or could be run direct into the precipitation tank (5, Pls. III and IV, B), depending upon whether it was desired to eliminate the iron, and thus obtain a higher-grade product. If the solution was not filtered, the oxide finally obtained was about 87 per cent U3O8. If the solution was filtered at this stage, the grade of the final product could be greatly increased, reaching as high as 95 per cent.
The solution was then run into tank 5 (Pls. III, and IV, B), which contained about 5,000 pounds of water, and an amount of ammonia slightly in excess of that required to neutralize the acid. After 300 pounds of sodium uranate had been treated in the manner described and rim into tank 5, the liquid in this tank was boiled for about two hours. It was then filtered through press 33c (Pl. IV), and the filtrate discarded. If this method were used continuously on a commercial scale, it would, of course, pay to treat the solution and recover the ammonia so that it could be used again.
The washed precipitate was then removed from the press, dried, and ignited in an iron pot in one of the oil furnaces in the furnace room. In this manner, the ammonium uranate was converted into oxide. As the product obtained, as already described, consisted of a mixture of uranium oxide and sodium uranate, it was treated with a 5 per cent solution of sulphuric acid, the sodium uranate dissolving along with a small amount of oxide. The solution was run into tank 5 and retreated.
Small-Scale Tests of Other Processes
Although this process worked fairly well, the costs were altogether too large to justify its permanent use. Other methods were therefore sought whereby a high-grade oxide could be obtained and the vanadium eliminated at the same time, with a decrease in the cost of conversion. In other words, a method was required by which uranium oxide could be cheaply and efficiently produced from the sodium uranate without any preliminary treatment to get rid of the 7 or 8 per cent of V2O5 always present. Following a suggestion made to one of the authors it was found that when sodium uranate was ignited in a platinum crucible to a temperature approaching 1,500° C., a certain part of the uranate was converted into oxide. During the heating, fumes of sodium oxide were evolved, and when the product obtained was leached with water, the water was found to be strongly alkaline and contained practically all of the vanadium that was in the sodium uranate. Some experiments were tried on a semicommercial scale, a small oil furnace containing a shelf of fire-clay on which the sodium uranate was placed being used. The flame was applied not only underneath the shelf but also over it, giving a more or less reverberatory effect. A temperature approaching 1,500° C. was obtained, and a considerable amount of sodium uranate was converted into oxide, but the conversion was by no means complete. In addition, the fire clay was attacked by the semiviscous mass. The conclusion was then reached that the only possibility of success along this line was to use an electric furnace.
Tests of Various Crucible Materials
It was also important to find some material that would not be seriously affected by the action of the melted sodium uranate. Among others, the following were tried:
An Acheson graphite crucible brasqued with magnesite; a similar crucible brasqued with alundum; a similar crucible brasqued with 50 parts of magnesite and 50 parts of alundum; a crucible made of silicon-carbide tubing bonded with carbon, with a graphite bottom luted in place with silicon carbide and water glass, and the inside of the crucible coated with silicon carbide and water glass; a crucible similar to the last except that quartz tubing was put in with the charge.
Alundum proved to be rather unsatisfactory, as it was quickly attacked. The magnesite lining separated fairly well from the charge, but the charge was to some extent contaminated. The charge also stuck badly to the lining of silicon carbide and water glass, and the result was that practically none of these materials appeared to be satisfactory. The experiments were carried on in a resistance furnace, with a granular carbon resistor in which the crucibles, or tubes, were embedded.
In another experiment 200 grams of sodium uranate was placed in a small Acheson graphite crucible with a direct arc from a carbon rod, constituting a small Heroult type furnace. In this, the whole charge fused, and copious clouds of alkaline fumes were given off. The product was black, and the vanadium could be readily leached out when the ground material was boiled with water. Similarly an iron pot cooled with water externally was used as one electrode and container, the other electrode being made of Acheson graphite. With this equipment several pounds of a fairly satisfactory black oxide was obtained. The results of this experiment were so encouraging that work was continued along this line in the plant.
Construction of Experimental Electric Furnace
The mechanical part of the experimental furnace consisted of a heavy 4-inch L-beam 10 feet long placed horizontally 8 feet from the floor, one end being embedded in the wall and the other bolted to a 3 by 4 inch wooden support embedded at its lower end in the cement floor. At the middle of this horizontal beam, two plates, or hangers, 2 feet by 4 inches by ½ inch were suspended. In the lower ends of these a 1-inch shaft carrying the sprocket wheels was placed horizontally. Over one sprocket wheel passed a chain having one end connecting with the electrodes and the other end off to one side passing over a wheel and terminating in a counterbalance approximately the same weight as the electrodes. Over the other sprocket wheel on the shaft in the central hanger passed a continuous chain about 10 feet long, running also on a sprocket wheel placed on a 1-inch shaft attached to the upright wooden support of the main horizontal L-beam. Attached to this 1-inch shaft was a 22-inch hand wheel, the turning of which raised and lowered the electrodes.
On the floor directly beneath the electrodes was placed a cement basin about 2½ feet square and 10 inches high, sloping inward toward the bottom and drained by a 2-inch pipe. For melting the sodium uranate an iron pot standing on legs was placed in the basin. The pot was cooled by water from a ring placed about 2 inches from the top and having 1/16-inch holes every half inch. About 3 inches of water was allowed to back up into the basin to cool the bottom of the pot. Around the inside of the basin, stones and bricks were packed loosely about halfway up the pot to minimize danger from an explosion, if any of the molten material should be thrown out of the pot. These loose stones and bricks were kept away from the pot by a sheet-iron sheath around the pot, being enough larger to allow free play for the cooling water to run down the sides of the pot.
The voltage first used was 170 volts, but after several trials the arc was found to be too heavy and to sputter too much. Other voltages ranging from 80 to 130 were tried, 130 volts proving the most satisfactory, although a 110-volt pressure was used to some extent. Leads of 600,000 circular mils about 20 feet long extended from the transformers, which were not sufficiently flexible to be connected to the electrodes, so that they were spliced to 5-foot lengths of flexible leads, ½ inch by 2 inches in cross section, made of soft-copper strips wrapped with tape.
In order to start the arc, it was found that the best method was to use a piece of uranium oxide from a previous charge, which was heated in an oil blast to bright redness and placed beneath and in contact with the electrodes. Such material is sufficiently conducting to melt and form the desired pool.
Melting Pots Used
The preliminary work was done in cast-iron pots, which, however, did not stand up under the treatment, and usually, after one or two runs, were melted through at some point. In order to make the pots last longer, a coating of fire clay on the inside was tried, magnesite being afterward substituted. The molten material, however, attacked, and to some extent dissolved both the fire clay and the magnesite; consequently, their use was abandoned. Finally, the plan already described was adopted, namely, the use of a heavy pot made of 3/8-inch boiler plate 14 inches in diameter and 10 inches on the side, with a round bottom, dipped 2 inches, and standing on legs, so that the lowest point of the bottom stood half an inch from a level floor. The pot was riveted and worked well when it was cooled by a stream of water from a circular pipe that surrounded it, as already described. The pot would hold about 160 to 175 pounds of product, but was unwieldy to remove and dump, so ladling was resorted to at intervals. This resulted in a yellow coating appearing on the material, which was probably re-formed sodium uranate, increasing the percentage of this impurity in the product. In order to eliminate this trouble as much as possible, a second pot was made, and after the charge had cooled the pot with contents was removed and the second one substituted.
The product finally obtained consisted of UO2, containing small traces of undecomposed sodium uranate, especially on the top and sides of the charge, and also some Na2O that had not been vaporized. After the product had been put through a ball mill and leached with water, the filtrate contained most of the vanadium that had been in the original sodium uranate, and this was recovered in the usual way. The leaching also washed out the soda which was present.
In order to eliminate the trace of sodium uranate, it was necessary to wash with dilute sulphuric acid, sodium uranate being soluble in a 4 or 5 per cent acid, whilst the oxide is not. Uranium oxide (UO2) is much less soluble not only in sulphuric acid but also in hydrochloric acid than U3O8, the amount of oxide going into solution with this treatment being small.
Explosions in Electric Furnace
Explosions in the melt, which quickly developed in connection with the use of the electric furnace, finally forced its abandonment. The explosions were of two distinct types—one like a sharp pistol crack, the other of full, round, cannon-boom character. The first frequently occurred alone, but the latter was always preceded by a sharp explosion. The sharp explosions evidently came from the melt, as they always threw molten material from the pot. At times the furnace could be operated for a considerable period smoothly and satisfactorily, when suddenly loud “cracks” would be heard, followed by molten material being thrown in all directions. Sometimes these cracking explosions would occur every 15 or 20 minutes. At first the authors were rather inclined to believe that the explosions were due to nitrate left in the sodium uranate, but later experiments with material washed perfectly free from nitrate gave the same results. Two or three of the explosions were violent enough to tear the hood from the bolts, break the “transite” asbestos boards, and throw the bricks out of the cement basin.
The possibility of the “booming” explosions being due to an explosion of carbon monoxide formed from the electrodes was investigated. Samples of gas were taken from just over the pot at several intervals and analyzed. In each sample mere traces of carbon monoxide or hydrogen were present; consequently, this explanation had to be abandoned. In addition, the draft through the hood was so excellent that there was no possibility of an accumulation of a sufficient amount of gas to cause the explosions. It is possible that they were caused by the sudden oxidation of Na or Na2O, but the authors were unable, under the conditions, to establish that theory. Although the operators were well protected by screens, operation of the furnace finally became so dangerous’ that its use was abandoned and the furnace was dismantled. Further work along this line, however, should be of value.
Cost of Treatment with Electric Furnace
Only a rough approximation of the cost of such treatment can be given, as the work was of purely experimental character. If the explosions could be eliminated, somewhere around 75 pounds of finished product per hour could be obtained with a pressure of about 110 volts and a current of about 600 amperes or about 65 kilowatt-hours. The electrodes wore away at the rate of about 3 inches per 100 pounds of output, and as only about one-half or less of a 30-inch electrode could be utilized, two electrodes would be needed for every 400 to 500 pounds of product.
After the abandonment of the electric furnace, experimental work was continued in an entirely new direction, and finally a method was evolved that appears satisfactory. The details of this method will be described in a later report of the Bureau of Mines.