US2984605A

US2984605A - Deposition of boron from fused salt baths

Info

Publication number: US2984605A
Application number: US799460A
Authority: US
Inventors: Hugh S Cooper
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-03-16
Filing date: 1959-03-16
Publication date: 1961-05-16
Anticipated expiration: 1978-05-16
Also published as: CH388278A

Description

United States Patent DEPOSITION OF BORON FROM FUSED SALT BATHS Hugh S. Cooper, Shaker Heights, Ohio, assignor to Walter M. Weil, Shaker Heights, Ohio No Drawing. Filed Mar. 16, 1959, Ser. No. 799,460 12 Claims. (Cl. 20460) This invention relates to new and improved processes for forming adherent elemental boron coatings on metal surfaces by fused salt bath electrolysis of boron oxide. Throughout this specification and the appended claims, the term boron oxide" is intended to mean the sesqui- Oxide, B203.

The production of elemental boron by the electrolysis of fused salt baths has been employed successfully on a commercial scale ,for a number of years. One such process, in which the fused salt bath contained potassium chloride or fluoride, boron oxide (B and potassium fiuoborate, is the subject of my Patent No. 2,572,249. Another such process, in which the fused salt bath contained only potassium fiuoborate and potassium chloride, is the subject of my Patent No. 2,572,248.

At the time of filing my applications for those patents, and even up to the present time, it has generally been believed, as pointed out in Patent No. 2,572,249, that, without potassium chloride or fluoride in the bath, potassium fiuoborate would undergo slow thermal decomposition, and the bath would become viscous and gummy.

For example, I had previously found that baths, con taining about one-third boron oxide with the remaining two-thirds being potassium fluoborate, were very viscons; and when electrolysis was attempted, the electrical resistance was extremely high and the current-carrying capacity was quite low. Also, the high operating temperatures, then believed to be required, caused a considerable loss of boron oxide by vaporization and fuming. In view of all of these difliculties, I concluded that successful electrolysis of fused baths containing potassium fiuoborate and boron oxide could not be achieved unless potassium chloride or fluoride was also present in the baths.

Surprisingly, it has now been discovered in accord ance with the present invention that, under certain conditions, tightly adherent boron coatings may be produced by the electrolysis of a fused salt bath consisting essentially of boron oxide and potassium fluoborate, without any potassium chloride or potassium fluoride being present in the bath. In addition, it was surprising to find that the employment of this bath provides a number 1 of advantages not afliorded by the processes of my earlier patents. For example, boron may be deposited on the lcathode as a permanent, adherent plate. Moreover, if the process is employed as a means for producing elemental boron, longer electrolysis periods between cathode changes are possible since the cathode coating is more dense and adheres tightly permitting thicker deposits to be retained on the cathode. In addition, the bath may be operated at temperatures of about 100 to 200 C. below those required for satisfactory use of the baths of my two prior patents. This is directly contrary to my prior conclusion that higher bath temperatures would be required with the components of the present bath system.

One of the conditions required to achieve successful electrolysis of the bath of the present invention is that the boron oxide in the bath be maintained below about by weight. When the boron oxide is maintained below this percentage, the viscosity of the bath is substantially reduced and the conductivity is greatly improved. Also, the loss of boron oxide due to fuming drops very sharply and the required minimum operating temperature of the bath during electrolysis is appreciably lowered. The conductivity appears to improve further when the boron oxide content is reduced to about 15%, reaching a maximum conductivity at about 10%. As a result, the initial boron oxide content in the bath is preferably maintained between about 2 or 3% and 15% by weight, and any additions of make-up boron oxide during the process are preferably limited to amounts which will not increase the boron oxide concentration above about 15% by weight.

The bath temperatures during electrolysis, when employing the present invention, may suitably be maintained in the range of about 500 and 1000 C., al though highest efliciency is achieved when the baths are operated at temperatures between about 550 and 750 C., and particularly between about 600 and 700 C. The latter temperature ranges are more than C. lower than the bath temperatures generally preferred for most efiicient operation with my prior bath systems. This means that less heat is required to bring the bath up to operating temperature initially and to maintain this temperature during electrolysis. The lower operating temperature also results in longer crucible life, particularly where graphite crucibles are employed, since the dissolution of graphite by the fused bath is accelerated at the higher temperatures.

During electrolysis, the boron oxide is electrolyzed to deposit elemental boron on the cathode and release oxy gen at the anode. The potassium fiuoborate is believed to act only as an electrically conductive inert vehicle, rather than as a source of boron as in the process of my Patent No. 2,572,248. in that prior process, the bath consists essentially of potassium fiuoborate and potassium chloride, with the fiuoborate being electrolyzed to deposit elemental boron and to form potassium fluoride in the bath while releasing chlorine from the anode.

Since the electrolysis products formed in the process of the present invention, namely, elemental boron and oxygen, do not remain as a part of the bath, the only change in the bath is a depletion of the boron oxide. Thus, substantially continuous operation may be achieved by the introduction of additional quantities of boron oxide into the bath to replenish that being removed by electrolysis. Since periodic cathode removal is necessary in this case for recovery of the deposited boron, small quantities of potassium fiuoborate should also be added occasionally to replace fluoborate which is lost due to entrainment with the boron deposit on the cathode, or by evaporation from the surface of the bath.

When the hot cathode having the boron deposit thereon is removed from the fused bath, a thin coating of the fused bath salts clings to the surface. The salts quickly solidify to form a coating which protects the surface of the boron from oxidation during cooling. If desired, the cathode may be rapidly cooled in a refrigerated atmosphere of an inert gas such as argon. In small operations, it may be more convenient to provide additional protection against oxidation, for example, by coating the cathode with an inert material such as cold sodium chloride salt. After cooling, the withdrawn cathode is washed with water to remove the outer coating of solidified salts while leaving the deposited boron and the bulk of the solidified salts entrained within the cathode deposit adhering tightly to the cathode as a hard, porous composite coating. If removal of the boron deposit from the cathode should be desired, the cathode with its adherent deposit may be soaked in boiling water to loosen the deposit from the cathode while dissolving most of the remainder of the entrained bath salts. Some of the cathode deposit may fall from the cathode while this soaking in boiling water continues, and the remainder may be readily knocked loose. The purity of the elemental boron recovered from the removed cathode deposit may be improved by leaching the deposit with strong hydrochloric acid, followed by a Water wash to remove all traces of the acid.

The process of the present invention is particularly useful where it is desired to plate a thin, permanently adherent coating onto a metallic object. Boron coatings may be deposited on objects made of metals such as iron, copper, molybdenum, nickel and other metals and alloys which are inert to the fused bath salts by using the metal objects as cathodes. The coatings or plates formed on the objects may be of any desired thickness, although for most purposes, thickness in the range of about 0.01 and 0.025 inch generally provide improved resistance to oxidation at elevated temperatures and provide improved resistance to abrasion. Although the boron coatings generally contain, dispersed through the coatings, a substantial proportion of entrained bath salts, these salts may be largely removed by vacuum distillation or cold Water leaching, if their presence is detrimental for a particular application, without loosening the coating on the cathode.

One important use for the boron coated products of the present invention is in applications where protection against atomic radiation is desired. Since boron normally contains approximately 20% of the B isotope, which is highly opaque to neutrons emitted by radioactive materials, sheets coated according to the method of the invention may be employed for radiation shielding purposes. The neutron shielding capacity of the coatings may be greatly improved if the boron coatings are produced by employing, in the fused bath, boron oxide in which the boron has been enriched to provide a greater percentage of the B isotope.

Various aspects of the invention will be more fully understood from the following examples and the accompanying discussion.

Example I About 16 kilograms of potassium fluoborate and 800 grams of boron oxide were melted together in a graphite crucible of the type described in my Patent No. 2,572,- 249 to produce a fused bath containing about 5% boron oxide. The graphite crucible was the anode and the cathode was an Armco iron plate (an iron alloy containing approximately 0.12% carbon, 0.017% manganese, 0.005% phosphorus, 0.025% silicon, and the balance, iron). After the melt reached a temperature of about 700 C., the current was turned on to place the cell in operation. Thevoltage ranged from about 7.4 to 7.7 volts while the current was substantially constant at about 700 amperes per square foot of cathode surface throughout the run.

After about one hour of electrolysis, an additional 454 grams of potassium fluoborate were added, reducing the boron oxide content in the bath down to about 3%, and the electrolysis continued for one hour more after which the cathode was removed. The hot cathode with the boron deposit thereon was quickly coated with sodium chloride salt to prevent oxidation of the boron by contact with the air during cooling. The elemental boron deposited on the cathode was very hard and adhered tightly to the cathode. To recover the boron in the cathode deposit, the cathode was soaked in boiling wa ter for several hours to loosen the coating. 140 grams of partially purified elemental boron were recovered from one stripping of the cathode.

Thereafter, a new cathode was inserted in the bath, and 900 grams of potassium fluoborate and 800 grams of boron oxide were added to the molten bath, raising the boron oxide content to about 6%. Electrolysis was started again with the voltage ranging between about 7.2 and 7.6 volts and the current remaining steady at about 700 amperes per square foot. After two hours, an

additional 300 grams of boron oxide were added to the molten bath to compensate for the amount decomposed during electrolysis, and electrolysis was continued for 1%. hours more, whereupon the current was turned oil and the second cathode removed. The deposit on the second cathode was hard and tightly adherent and similar to that obtained during the first run. 265 grams of partially purified elemental boron were recovered from a single stripping of the cathode by the procedure employed to recover the first cathode deposit.

The following day, the bath remaining from the previous electrolysis runs was remelted and about 2700 grams of potassium fluoborate and 1000 grams of boron oxide were added to bring the boron oxide content up to about 7%. When the bath temperature reached 700 (1., a new cathode was inserted and the current was turned on. The temperature was maintained during electrolysis between about 660 and 700 C. The voltage ranged from about 7.3 to 8.6 volts with an average current of about 600 amperes per square foot. After four hours, 600 grams of boron oxide were added to the molten bath to compensate for the amount decomposed during electrolysis, and at the end of two more hours, the run was terminated. The cathode deposit was very hard and adhered tenaciously to the cathode as did the cathode deposits above. The elemental boron recovered as before from a single stripping of the cathode weighed 480 grams.

The production of the three runs above was combined and the resulting 885 grams of elemental boron washed with water, with hydrochloric acid, and with water again, to remove the entrained bath salts. The resulting product analyzed 98% boron, 0.64% iron, and 0.20% carbon. After being arc melted, the product analyzed 98.52% boron, 0.66% iron, and 0.35% carbon.

Example 11 About 18 kilograms of potassium fluoborate and 2 kilograms of boron oxide were melted together in a graphite crucible of the type described in Example I to form a molten bath containing approximately 10% boron oxide. After the melt reached a temperature of about 700 C., electrolysis was started using an Armco iron plate as the cathode and the graphite crucible as the anode. The voltage was about 7 volts, and the current was about 400 amperes per square foot. After 15 minutes, the current was shut off and the cathode removed, cooled in an atmosphere of argon, and then rinsed with water. A thin, hard, porous plate of boron (0.065 inch thick) had been deposited on the cathode. The tenacity of the plate was tested by striking the cathode with a steel hammer. Although the hammer impressions were visible on the surface, the plate did not crack off the cathode but remained tightly adherent thereto.

Example III A new cathode was placed in the electrolytic bath of Example II, and electrolysis was begun under the same conditions of bath temperature, voltage, and current. At the end of one hour, this cathode was removed and cooled in argon. After being rinsed with water, the plate on the cathode was measured and found to be 0.197 inch thick. The tenacity of the plate was tested as in Example II by striking with a hammer, and the plate was found to be as tightly adherent to the cathode as the plate in Example II.

Example IV The procedure of this example was the same as Example II except that the proportion of boron oxide in the bath was increased to 20%. After a 15 minute electrolysis period at about 7 volts and an average current of about 600 amperes per square foot, a thin, hard, porous, tightly adherent plate 0.061 inch thick similar to that produced in Examples II and III, was deposited on the cathode.v

Example V A new cathode was placed in the electrolytic bath of Example IV and electrolysis carried out under the same conditions. After an electrolysis period of one hour, a hard, porous, tightly adherent plate 0.155 inch thick, similar to that produced in Examples II, III, and IV, was deposited on the cathode.

From the foregoing, it will be appreciated that the present invention provides a new and improved process for producing tightly adherent boron coatings on a cathode by electrolysis of a fused bath consisting essentially of boron oxide and potassium fluoborate. Not only does the process of the invention successfully solve the problems that previously prevented successful electrolysis of fused baths of boron oxide and potassium fluoborate, but also, the process provides a number of advantages not heretofore attainable. The boron may be deposited on the cathode as a permanent, tightly adherent plate. Moreover, when the deposit is to be removed from the cathode for recovery of elemental boron, longer electrolysis periods between cathode changes are possible since the cathode coating is more dense and adheres more tightly, permitting thicker deposits to build up on the cathode. In addition, the process of the invention may be operated eificiently at bath temperatures more than 100 below those commonly used heretofore.

It is apparent from the above description of the invention that various modifications of the process as described can be made within the scope of the invention. Therefore, the invention is not intended to be limited to the details of the specific processes disclosed herein except as may be required by the appended claims.

What is claimed is:

l. A process for producing boron from boron oxide, which comprises electrolyzing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 20% by weight of the boron oxide.

2. A process for producing boron from boron oxide, Which comprises electrolyzing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and by weight of the boron oxide.

3. A process for producing a permanent boron coating on a metallic object, which comprises electrolyzing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15% by weight of boron oxide in an electrolytic cell in which said object is the cathode, and removing the cathode from the cell after a coating of boron has been deposited thereon.

4. A process for producing boron from boron oxide, which comprises electrolyzing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15% by weight of the boron oxide in an electrolytic cell having a removable metal cathode, removing the cathode from the cell, and recovering boron deposited thereon.

5. A process for producing boron from boron oxide, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and by weight of the boron oxide, electrolyzing said bath in an electrolytic cell while maintaining the temperature of the bath in the range of about 500 to about 1000 C.

6. A process for producing boron from boron oxide, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15% by weight of the boron oxide, electrolyzing said bath in an electrolytic cell while maintaining the temperature of the bath in the range of about 550 to about 750 C.

7. A process for producing a permanent boron coating on a metallic object, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15% by weight of boron oxide, electrolyzing said bath in an electrolytic cell in which said object is the cathode, maintaining the temperature 70 6 of the bath in the range of about 500 to about 1000 C., during electrolysis, forming a tightly adherent boron deposit on the cathode, and removing the cathode from the cell after a coating of boron has been deposited thereon.

8. A process for producing boron from boron oxide, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15% by weight of the boron oxide, electrolyzing said bath in an electrolytic cell having a removable metal cathode while maintaining the temperature of the bath in the range of about 500 to about 1000 C., forming a tightly adherent boron deposit on the cathode, and removing the cathode from the cell and recovering boron deposited thereon.

9. A process for producing boron from boron oxide, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 20% by weight of the boron oxide, electrolyzing said bath in an electrolytic cell having a removable metal cathode while maintaining the temperature of the bath in the range of about 500 to about 1000 C., and adding boron oxide to the bath to replace that consumed during the electrolysis.

10. A process for producing boron from boron oxide, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15% by weight of the boron oxide, substantially continuously electrolyzing said bath in an electrolytic cell having a removable metal cathode while maintaining the temperature of the bath in the range of about 500 to about 1000 C., forming a tightly adherent boron deposit on the cathode, replenishing the boron oxide as needed to maintain the boron oxide concentration betweeen about 2% and 15% by weight, periodically removing the cathode from the cell, and replacing the removed cathode with a new cathode to permit continuation of the electrolysis.

11. A process for producing permanent boron coatings on metallic objects, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15 by weight of boron oxide, substantially continuously electrolyzing said bath in an electrolytic cell having at least one of said objects as the cathode while maintaining the temperature of the bath in the range of about 550 to about 750 C., forming a tightly adherent boron deposit on the cathode, replenishing the boron oxide as needed to maintain the boron oxide concentration in the bath between about 2% and 15% by weight, periodically removing one of said objects from the cell and replacing the removed object with an uncoated one to permit continuation of the electrolysis.

12. A process for producing boron from boron oxide, which comprises preparing a fused salt bath consisting essentially of potassium fluoborate and between about 2% and 15 by weight of the boron oxide, substantially continuously electrolyzing said bath in an electrolytic cell having a removable metal cathode while maintaining the temperature of the bath in the range of about 550 to about 750 C., forming a tightly adherent boron deposit on the cathode, replenishing the boron oxide as needed to maintain the boron oxide concentration between about 2% and 15% by weight, periodically removing the cathode from the cell for recovery of the boron deposited thereon, replacing the removed cathode with a new cathode to permit continuation of the electrolysis, and replenishing the potassium fluoborate as needed to maintain an elfective quantity of the fused bath in the cell.

References Cited in the file of this patent UNITED STATES PATENTS 2,572,249 Cooper Oct. 23, 1951 2,810,683 Ellis Oct. 22, 1957 2,848,396 Murphy et al Aug. 19, 1958

Claims

1. A PROCESS FOR PRODUCING BORON FROM BORON OXIDE, WHICH COMPRISES ELECTROLYZING A FUSED SALT BATH CONSISTING ESSENTIALLY OF POTASSIUM FLUOBORATE AND BETWEEN ABOUT 2% AND 20% BY WEIGHT OF THE BORON OXIDE.