6.1 Electrolysis of Water

History of process. Until 1943, all the heavy water produced commercially was made by electrolysis. The largest single producer of heavy water was the Norsk Hydro Company, which operated the world’s largest electrolytic hydrogen plant at Rjukan, Norway. In 1942, this plant was making about 1.5 MT of heavy water per year as a by-product of the production of 17,300 nm3 of electrolytic hydrogen per hour, used for ammonia synthesis. The average power consumption of this plant was 91,000 kW, or 5.2 kWh/nm3 of hydrogen.

The primary plant at Rjukan made water containing 15 a/о deuterium. The electrolytic cells were of the Pechkranz [М2] type, with steel cathodes and diaphragms to prevent mixing of hydrogen and oxygen. Nine stages of parallel-connected cells were used, with the number of
і Figure 13.11 Characteristics of Sul — zer CY packing for water distilla­tion service.

cells per stage decreasing as the deuterium content increased. The stages were connected in a series cascade, without recycle of partially enriched hydrogen, and the cascade was operated in steady flow. A schematic flow sheet for this kind of plant is shown in Fig. 13.13. About 73 percent of the water fed to each stage was electrolyzed; and 27 percent was carried from the stage by the products of electrolysis as water vapor, condensed, and fed to the next higher stage of the cascade. The fraction of water fed forward was controlled by the vapor pressure of water; 27 percent forward feed requires an electrolyte temperature of 60°C.

The product of the primary plant was refined to pure D2 О in a small, nine-stage secondary plant, also operated with steady flow, but with the partially enriched hydrogen burned and recycled, as shown in Fig. 13.14. The secondary electrolytic plant has since been replaced by a water distillation plant.

During World War II heavy-water production at Rjukan was increased by addition of steam-hydrogen deuterium exchange equipment, to be described in Sec. 7. In 1943 operation was interrupted by a series of commando raids, but production was resumed after the war and was at the rate of 6.5 Mg/year in 1975 [R3]. A second electrolysis and exchange plant at Glomfjord, Norway, was then producing 5.9 Mg/year.

The steady-flow electrolytic process without recycle, shown in Fig. 13.13, was also used at the plant of Emswerke AG at Ems, Switzerland, to produce 400 nm3/h of hydrogen enriched

Figure 13.12 Dostrovsky’s [D4] water distillation plant for concentration of 180.

sixfold in deuterium over natural abundance [H3] and is being used at the Indian government’s fertilizer plant at Nangal, India, to produce 5,000 nm3/h of hydrogen containing 3.1 times the natural abundance of deuterium [Gl]. In each case the partially enriched hydrogen goes to a hydrogen distillation plant for final concentrations of deuterium, as was described in Sec. 13.4.

At the Manhattan District’s heavy-water plant at Trail, British Columbia, primary concen­tration of deuterium was effected by the combination of electrolysis and steam-hydrogen

exchange, to be described in Sec. 7. The plant made use of hydrogen produced electrolytically by the Consolidated Mining and Smelting Company for ammonia synthesis. This was the largest electrolytic hydrogen plant in North America. In 1945 the average hydrogen production rate was 14,000 nm3/h, almost as great as at Rjukan. The electrolytic cells used at Trail have been described by Mantell [М2]. The cells were operated with steady flow. Because the principal means for isotope separation in the primary plant at Trail was by the exchange process rather than by electrolysis, no special efforts were made to obtain a high separation factor in the primary plant. .

The electrolytic process was also used by the Manhattan District, at Morgantown, West Virginia, and at Trail, British Columbia [M8], to refine crude heavy water from a primary plant where some process other than electrolysis was employed. These electrolytic plants were operated batchwise. The cells had no diaphragm, so the product was a mixture of hydrogen and oxygen. The gases were recombined in a burner, and the water was recycled to the primary plant when its deuterium content was leaner than primary-plant product or to the next batch of the electrolytic plant when its deuterium content was richer than primary-plant product.

Details of the Manhattan District’s secondary electrolytic plants are given by Maloney et al. [М8].

Batch electrolysis was used to concentrate deuterium from 90 to 99.87 percent at the large Savannah River heavy-water plant of the U. S. Atomic Energy Commission, at Aiken, South Carolina [B7, B8], but this final concentration step is not needed when the plant is operated at reduced capacity.

Separation factors. Deuterium separation factors in the electrolytic plants described above, together with the types of cells used and operating conditions that may have had an effect on separation factor, are listed in Table 13.13. Separation factors of from 6 to 10 have been reported for the secondary plants, and from 3.8 to 7.0 for the primary plants. The lower values for the primary plants are attributed to their higher cell temperatures, their use of diaphragms, and the greater difficulty of keeping large equipment clean.

In a detailed laboratory investigation of the effect of cell variables on the deuterium separation factor in electrolysis of water, Brun and co-workers [ВІЗ] have found that a depends on the cathode material, electrolyte composition, and cell temperature, generally as follows. The separation factor is higher for an alkaline electrolyte than for an acid. With KOH, at 15°C, a pure iron cathode gave the highest value reported, 13.2. The separation factor for mild steel, the material used in most commercial electrolyzers, was 12.2. Values as low as 5 were reported for tin, zinc, and platinized steel. At 25°C the separation factor with a steel cathode was 10.6, and at 75°C it had dropped to 7.1.

Because the equilibrium constant for the reaction

H2 O(0 + HDfe) «* HDO(0 + H2 (?)

is 3.81 at 25°C and 2.95 at 75°C, it is evident that the much higher separation factors obtained in electrolysis must be due to some mechanism other than establishment of equilibrium in this reaction at the cathode surface. One plausible explanation is that the hydrogen ion is discharged more readily at the cathode than the deuterium ion.