2 History of the Chlor-Alkali Industry During the last half of the 19th century, chlorine, used almost exclusively in the textile and paper industry, was made [1] by reacting manganese dioxide with hydrochloric acid 100–110◦C MnO2 + 4HCl −−−−−−→ MnCl2 + Cl2 + 2H2O (1) Recycling of manganese improved the overall process economics, and the process became known as the Weldon process [2]. In the 1860s, the Deacon process, which generated chlorine by direct catalytic oxidation of hydrochloric acid with air according to Eq. (2) was developed [3]. ◦ 450–460 C;CuCl2 cat. 4HCl + O2(air) −−−−−−−−−−−−−−→ 2Cl2 + 2H2O(2) The HCl required for reactions (1) and (2) was available from the manufacture of soda ash by the LeBlanc process [4,5]. H2SO4 + 2NaCl → Na2SO4 + 2HCl (3) Na2SO4 + CaCO3 + 2C → Na2CO3 + CaS + 2CO2 (4) Utilization of HCl from reaction (3) eliminated the major water and air pollution problems of the LeBlanc process and allowed the generation of chlorine. By 1900, the Weldon and Deacon processes generated enough chlorine for the production of about 150,000 tons per year of bleaching powder in England alone [6]. An important discovery during this period was the fact that steel is immune to attack by dry chlorine [7]. This permitted the first commercial production and distribu- tion of dry liquid chlorine by Badische Anilin-und-Soda Fabrik (BASF) of Germany in 1888 [8,9]. This technology, using H2SO4 for drying followed by compression of the gas and condensation by cooling, is much the same as is currently practiced. 17 “chap02” — 2005/5/2 — 09Brie:49 — page 17 — #1 18 CHAPTER 2 In the latter part of the 19th century, the Solvay process for caustic soda began to replace the LeBlanc process. The resulting shortage of HCl made it necessary to find another route to chlorine. Although the first formation of chlorine by the electrolysis of brine was attributed to Cruikshank [10] in 1800, it was 90 years later that the electrolytic method was used successfully on a commercial scale. In 1833, Faraday formulated the laws that governed the electrolysis of aqueous solutions, and patents were issued to Cook and Watt in 1851 and to Stanley in 1853 for the electrolytic production of chlorine from brine [11]. 2.1. DIAPHRAGM-CELL TECHNOLOGY DEVELOPMENT During the electrolysis of NaCl brine, chlorine is generated at the anode and sodium hydroxide is produced at the cathode. – At the anode: 2Cl → Cl2 + 2e – At the cathode: 2H2O + 2e → H2 + 2OH 2F Overall reaction: 2NaCl + 2H2O −→ 2NaOH + Cl2 + H2 The main difficulty during the electrolysis of NaCl solutions was that of achieving continuous separation of chlorine generated at the anode and sodium hydroxide pro- duced at the cathode. While it was easy to keep the chlorine and hydrogen gases in U-shaped tubes, the sodium hydroxide formed at the cathode reacted with chlorine to form sodium hypochlorite. The British scientist Charles Watt devised the concept of a current-permeable separator, which allowed the electric current to pass but kept the anode and cathode products separated. Thus, the diaphragm cell was invented in 1851. The major drawback to the use of the Watt cell was the lack of electrical generation capacity. The development of the dynamo around 1865 allowed Edison, Siemens, Var- ley, Wheatstone, and others to invent generators of electricity with sufficient capacity and efficiency to make electrolytic production of chlorine and caustic soda feasible. Following the breakthroughs in electricity generation, parallel developments in dia- phragm cells were made in many countries. The credit for the first commercial cell for chlorine production goes to the Griesheim Company in Germany, in 1888. This nonper- colating diaphragm cell, used predominantly for Cl2 production in the early 1900s, was based on the use of porous cement diaphragms, invented by Brauer in 1886 and made by mixing Portland cement with brine acidified with HCl, followed by setting and soaking in water to remove the soluble salts. The cell, termed the Griesheim Elektron cell [12, 13] and shown in Fig. 2.1, consists of an iron box with a steam jacket and is mounted on insulation blocks. Each unit contains six rectangular boxes made of cement about 1 cm thick. The cement boxes act as diaphragms and contain the anodes made of magnetite or graphite. The outer box forms the cathodes, and cathode plates are also provided in the form of iron sheets placed between anode compartments and reach the bottom of the cell. The cell was operated at 2.5 kA, batchwise, with saturated potassium chloride solutions for 3 days until a concentration of 7% KOH was obtained, at a current density of 100–200 A/m2 at 80–90◦C; the cell voltage is about 4 V and the current efficiency 70–80%. It is very interesting to note that the best operating conditions for this cell were “chap02” — 2005/5/2 — 09Brie:49 — page 18 — #2 HISTORY OF THE CHLOR-ALKALI INDUSTRY 19 c ccc c A c c c c c B ba d s s e C FIGURE 2.1. A and B: Griesheim Elecktron cell bank; C: Cross-section of Griesheim Elecktron cell. a: anode; b: brine inlet; c: cathode; d: brine outlet; e: caustic outlet; s: steam. found to be: (1) anodes preferably of magnetite, to obtain pure chlorine without any CO2 in the anode gas, (2) high concentration of brine, and (3) a temperature of 80–90◦C, to reduce the cell voltage. Items (2) and (3) are still the desired parameters for optimal cell operation. The first diaphragm cell developed in Great Britain was the Hargreaves–Bird cell, operated in 1890 by the United Alkali Company. Each cell consisted of a rectangular iron box lined with cement. The box was 10 ft long, 4–5 ft deep, and 2 ft wide, divided into “chap02” — 2005/5/2 — 09Brie:49 — page 19 — #3 20 CHAPTER 2 +A S S CC DD O O FIGURE 2.2. Hargreaves–Bird cell. A: anodes; C: cathodes; D: diaphragms; O: outlet pipes for brine mixed with sodium carbonate; S: outlets for CO2 and steam. three parts of two separate asbestos sheet diaphragms. Six carbon anodes were placed in the anode compartment, and the cathode was a copper gauze attached to the diaphragms. This basic approach of anchoring the diaphragm onto the cathode is still used in modern diaphragm cells. The cell design is shown in Fig. 2.2. The anode compartment was filled with saturated brine, and during electrolysis chlorine evolved at the anode. The sodium ions, along with sodium chloride and water, percolated through the diaphragm into the cathode compartment. Back migration of hydroxyl ions was suppressed by injecting CO2 and steam into the cathode compartment to form sodium carbonate. The major contribution of this cell was its vertical diaphragm configuration, which is the basis of modern cells. Twelve cells were run in series at 2 kA, which corresponds to a current density of 200 A/m2, at 4 to 4.5 V, when 60% of the salt is converted to sodium carbonate. The cell, used in France and Italy, was the Outhenin–Chalandre cell [13], which consisted of an iron box divided into three compartments; the outer ones contained the cathode liquor, and the inner one had the graphite anodes in strong brine. The diaphragm was made from cylindrical unglazed porcelain tubes, cemented together into dividing walls of the cell. This cell was more complex than the Griesheim cell and required considerable attention. In the United States, LeSueur [14–17] developed and operated a cell in 1890, employing a percolating horizontal diaphragm (Fig. 2.3A). This design permitted brine to flow from the anolyte through the diaphragm continuously to achieve a higher efficiency than the contemporary nonpercolating diaphragm cells in Germany and Great Britain. LeSueur’s percolating diaphragm cell, which is the basis for all diaphragm chlor-alkali cells in use today, shown in Fig. 2.3B, is an improved version of the cell depicted in Fig. 2.3A. The LeSueur cell was made of iron and divided into two compartments separated by a diaphragm, which was deposited on an iron-gauze cathode. The anode was graphite, and the anode compartment was sealed to avoid the release of chlorine. The liquid level in the anode compartment was higher than the level in the cathode compartment, and the caustic flowed out of the cell continuously. This was the first cell to use the percolation method for caustic withdrawal. “chap02” — 2005/5/2 — 09Brie:49 — page 20 — #4 HISTORY OF THE CHLOR-ALKALI INDUSTRY 21 Anode Anode Chamber Cathode Chamber Diaphragm Cathode A Cl2 Outlet Brine Inlet ++ Cathode Anode Chamber Chamber H2 Outlet Caustic Outlet Cathode Diaphragm B FIGURE 2.3. A: original LeSueur cell; B: modified LeSueur cell. During the 1920s, the Billiter cell was developed in Germany [13]. This cell was similar to the LeSueur cell, and the original design had a flat electrode and a horizontal diaphragm. By modifying the cathode to a corrugated shape, current loads were increased from 12 kA to 24 kA (Figs. 2.4A and B). The diaphragm was a closed woven steel screen covered with a mixture of long fiber asbestos wool and barium sulfate. Asbestos became the primary component of the diaphragm for the next 80 years. There was significant cell development activity during the period 1890–1910, following LeSueur’s and Hargreaves–Bird cell with a vertical diaphragm, and the devel- opment of synthetic graphite independently by Castner and Vaughn, and Acheson [18], who also developed a method for producing synthetic graphite anodes. The cell designs focused on heavy concrete housing, either cylindrical or vertical, with graphite anodes entering through the top and the cathode plates bolted on the side.