Mercury process

The amalgam cells consist of slightly inclined steel troughs, over the bottoms of which flow a thin mercury layer, which absorbs the sodium and acts as the cathode. Horizontal anodes adjustable in height at which chlorine is produced are incorporated into the lid of the cells. The chlorine is drawn off upwards through gas extraction slits. 

The amalgam emerging from the ends of the cells is converted on graphite into mercury, 50% sodium hydroxide solution and hydrogen in a strongly exothermic reaction (see Fig. 1.7-5, 1.7-6 and 1.7-7). 

A salt solution with a sodium chloride content of ca. 310 g/L is electrolyzed at ca. 80°C, during which the sodium chloride content falls to 260 to 280 g/L. This is then concentrated by adding solid salt and recycled. During electrolysis the following reactions take place  

The electrochemical yield is 94 to 97%, the energy consumption ca. 3300 kWh/t chlorine, the effective cell voltage 4.2 V and the current density 8 to 15 kA/m.  

The amalgam formed at the cathode is decomposed with water  

---

Early demand for chlorine centered on textile bleaching, and chlorine generated through the electrolytic decomposition of salt (NaCl) sufficed. Sodium hydroxide was produced by the lime—soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for chlorine for PVC manufacture led to the production of chlorine and sodium hydroxide as coproducts. Solution mining of salt and the avadabiHty of asbestos resulted in the dominance of the diaphragm process in North America, whereas soHd salt and mercury avadabiHty led to the dominance of the mercury process in Europe. Japan imported its salt in soHd form and, until the development of the membrane process, also favored the mercury ceU for production. 

Finally, it should be mentioned that three out of the eight Spanish chlor-alkali plants operating with the mercury process are located in the Ebro River basin in the cities of Sabinanigo and Monzon - along the tributaries Gallego and Cinca Rivers, respectively - and Flix along the Ebro River (Fig. 1). Indeed, mercury emissions from the Hix and Monzon have already been reported [28]. Therefore, the mid-low Ebro River watershed might be considered as a hot spot of aquatic pollution of mercury in Spain. 

Chlorine-alkali electrolysis is the largest application of such materials as these are the only materials that can be used successfully in this process. As this process provides alkali of better quality than the conventional diaphragm process, and is much more attractive environmentally than the mercury process, its part in industrial world manufacturing of alkali is expanding rapidly. 

It is for this reason that the Chief Executive Officers of every West European chlor-alkali producer have, through Euro Chlor, entered voluntarily into six binding commitments [15]. All but one of these are unconditional. In this sense they do not represent part of some kind of negotiation rather, they are an attempt to demonstrate by action that the industry, whatever its past, is now totally committed to addressing public concerns about its use of the mercury process. It matters not that these concerns are almost entirely without foundation they are there, and we have to address them. 

Remaining mercury cellrooms will close or convert to non-mercury processes when they reach the end of their economic lives. The exact date will depend on the availability of capital and on macroeconomic factors more under the control of governments than industry. All available independent analyses point to this equating to an end for the mercury process in Western Europe somewhere in the 2020s. 

Fig. 17.2 Unit energy consumption of membrane, diaphragm, and mercury processes.

Pliny the Elder (A.D. 23-79) said that grains of gold were found in the stream-beds of the Tagus in Spain, the Po in Italy, the Hebrus in Thracia, the Pactolus in Asia Minor, and the Ganges in India (5). In the second century before Christ, a cupellation process was used for refining the metal, and in Pliny s time the mercury process was well known (6). 

In the mercury process the bottom of the electrolytic cell is covered with a layer of mercury into which a non-porous diaphragm dips so that the mercury forms a partition between the anodic and cathodic chamber. The anode is made of carbon, and is immersed in sodium-chloride solution the cathode is made of iron, and is dipped into water. The mercury acts as cathode, taking up the liberated sodium to form an amalgam, which reacts with the water to produce sodium hydroxide. Various modifications of the process have been devised, one being the substitution of fused sodium chloride for the solution, and of fused lead or tin for mercury, the alloy produced being subsequently decomposed by water.10... 

In addition to the Hilbert-Johnson reaction, the so-called mercuri process,37 and, less frequently, the cyclization procedure of Shaw et a/.,53 64 have been used for the synthesis of nucleosides and their derivatives. l-Peracylglycosyl-4-alkoxy-2(l//)-pyrimidinones, the intermediates of the Hilbert-Johnson reaction, can be, in principle, prepared 56,56 also by the mercuri process, namely by reaction of 4-ethoxy-2(lZ/)-pyrimidinone chloromercuri salt with the corresponding halogenoses, but this method is of less importance because of the contamination of iV-l-glycosyl derivatives with the 0-2 isomers, namely, with 2-peracylglycosyloxy-4-alkoxypyrimidines. The advantageous features of the mercuri process in comparison with the Hilbert-Johnson reaction might be formulated as follows. 

The products of the mercuri process in most cases possess a C-V-C-2 -trans system in accordance to the Baker trans rule61-64 and are sterically more uniform than the Hilbert-Johnson products. On the other hand, certain anomers inaccessible by the mercuri process may be obtained under certain conditions (solvent) by the Hilbert-Johnson reaction as a mixture with the other anomer chromatographic methods have been worked out to separate these mixtures almost quantitatively into the individual anomers (cf. Section X). The Hilbert-Johnson reaction intermediates may be converted into nucleosides of both types, of the uridine type as well as of the cytidine type (cf. Section V), whereas conversion of the mercuri process of the uridine type into the cytidine type is somewhat tedious. On the whole, the Hilbert-Johnson reaction represents a suitable complementary method to the mercuri process. 

Scheme 93 Preparation of vinyl and isopropenyl chloroformates by the mercury process.

Several chemists teams in the world, as well as researchers in our laboratories attempted to reproduce this exciting simple and cheap process. Unfortunately, whatever the conditions and catalysts used, all the carried out trials failed and the only products isolated were mesityl oxide and various chlorinated compounds. The conclusion of most investigators was that Matuszak did not isolate isopropenyl chloroformate but a mixture of chlorinated products. However, as shown in table 3-13, the properties given by Matuszak closely correspond to those of the isopropenyl chloroformate made by the mercury process (see farther on in this section). 

Table 3-13 Comparison of properties of the product obtained by Matuszak and properties of the isopropenyl chlororoformate made by the mercury process.

Purification for the mercury process multistage precipitation with Ba, NaOH, NajCO,... 

Depending upon the electrolysis process utilized amalgam, diaphragm or membrane, different additional purification steps are required. In the mercury process, solid salt is utilized, which is dis.solved in water. If evaporated salt is used, purification can be carried out in a small branch loop. When mined salt is utilized, care has to be taken during dissolution to settle out the impurities. Soluble impurities are removed by precipitating S04 with Ba +, precipitating Mg + and Fe- as hydroxides by the addition of NaOH and precipitating Ca as carbonate with sodium carbonate (see the production of evaporated salt). 

The membrane process also utilizes solid salt, but with a much higher purity, particularly as regards multivalent ions. Thus the Ca + content has to be additionally reduced to below 0.1 ppm (compared with 3 ppm for the mercury process) with the aid of ion exchangers such as Lewatit TP 208 (see Section 1.7.2.3.3). 

Upon electrolysis, the sodium chloride content of an initially saturated solution falls to ca. 170 g/L. The reactions at the anode are the same as in the mercury process. However, hydrogen is produced at the steel cathode ... 

Recovery of sodium hydroxide The alkali solution is evaporated to 50% by weight of sodium hydroxide, whereupon the salt, except for a residual 1%, precipitates out. This salt is very pure and can be further utilized for concentrating depleted brine or, in the case of combined plants, in the mercury process. 

The electrical energy consumption is ca. 20% less than that in the mercury process. 

The brine has to be much purer than for the mercury process. Ca + content, for example, must be below 20 ppb, otherwise Ca(OH)2 precipitates in the membrane, rapidly leading to its destruction see Section 1.7.2.2). 

---