Amalgam Decomposers

There are two types of industrial decomposers, horizontal and vertical. Horizontal decomposers are ducts with rectangular channels (Fig. 5.12) located below the cells with a 1.0-2.5% slope. The amalgam flows with a depth of 10 mm, and the catalyst is in the form of graphite blades 4-6 mm thick, immersed in the amalgam. 

The amalgam reacts with demineralized water in counterflow. Horizontal decomposers require about 30-40% of the space required for the cells. Horizontal decomposers produce caustic with a low mercury content. However, they are difficult to service and maintain. They are obsolete and have been replaced by vertical decomposers. 

Vertical decomposers are towers packed with graphite spheres or particles 8-20 mm in diameter. A typical cross section is 0.35 m per l(X)kA of cell load. The amalgam flows from the top and water is fed into the bottom of the tower. Since the volume of the decomposer is small, it is necessary to cool the hydrogen generated during the course of the amalgam decomposition reaction. The mercury inventory is small with the vertical decomposer. However, the caustic contains more mercury. 

FIGURE 5.13. Modified decomposer of DeNora [72]. (With permission from DENORA ELETTRODI S.p.A., Milan.) 

---

Sodium amalgam containing about 0.2 wt % Na is decomposed with water in the amalgam decomposition tower (see Section 5.1.2 for details related to the design of the amalgam decomposer) of... 

Hydrogen from the cells is cooled by heat exchangers located at the top of the amalgam decomposer to recover mercury and caustic mist, and returned to the gas holder. The gas is cooled further, if necessary, prior to removal of trace mercury by adsorption using activated carbon and/or ion-exchange resin columns. 

A schematic diagram of an amalgam decomposer is shown in Fig. 15. Sodium amalgam flows down the tower at a rate of M (kgmoles/h) with a concentration of Yo (kg moles Na/Kg moles Hg) and water exits the tower at a rate of Lx (kg moles/h) with a concentration of (kg moles of NaOH/kg mole water). Obviously water is consumed in the electrochemical reaction resulting in H2 production at a rate of (Lo - LJ. Thus, the mass transfer, W, at a distance h may be represented as... 

These equations permit estimation of the required volume of the amalgam decomposer. However, the important factors involved in the design of a decomposer column are the height and the diameter of the column and not its volume. Height, H, can be calculated, following Chilton and Colburn s concept for packed columns,70 as... 

The three major steps in the amalgam chlor-aUcali process, shown in Fig. 4.9.1, are brine treatment, electrolysis, and amalgam decomposition. Purilied brine is sent to the electrolyzer to produce chlorine and sodium amalgam at the anode and cathode, respectively. The sodium amalgam is then sent to the amalgam decomposer to produce caustic soda of 50% concentration. Mercury, after decomposition, is recycled to the top of the cell. The overall process of amalgam decomposition can be described as ... 

FIGURE 4.9.8. HTU of the amalgam decomposer tower as a function of the flow velocity of mercury and the specific conductivity of the caustic soda solution, k refers to the conductivity of NaOH solution in mho m [ 1 ]. 

TABLE 4.9.1 An Example of Heat Balance in the Amalgam Decomposer... 

Copper or aluminum bus bars carry the electric current to the cell covers, and then by flexible copper straps to the anode rods. The short-circuiting switches are located beneath the cells and when the cell is short-circuited, the cell bottom also acts as a current conductor. A vertical amalgam decomposer is located at the end of each cell. 

The strength of the caustic produced in membrane cells depends on the type of membrane chosen. Since membranes ideally pass only water and alkali metal ions, chloride, chlorate, and sulfate in the caustic are measured in parts per million. The highest-grade caustic solutions (barring mercury contamination) are produced in amalgam cells. Through control of the rate of addition of water to the amalgam decomposer, these cells can also produce caustic directly at commercial concentrations (50% for NaOH and 45-50% for KOH). These differences result in the separate flow lines of Fig. 6.8. 

The reaction product is a slurry of crystals of Na2S204-2H20. The slurry is pumped through a filter press to remove the crystals. The filtrate is the sulfite solution that absorbs the SO2 as it recycles to the amalgam decomposer. Isolation of the anhydrous product involves dehydration of the crystals at 60-65°C, filtration, washing with alcohol, and drying under vacuum. The product may contain a small amount of residual mercury. This prevents its use in some applications. 

Grube s horizontal mercury cell more closely resembled the standard salt electrolyzers and had an asbestos diaphragm between the electrodes in order to isolate the amalgam cathode from sulfuric acid. This cell produced pure caustic soda free of chloride ions from its amalgam decomposer. However, voltage and energy consumption still was high ( 4,000 kWhr t NaOH). 

It reacts outside of the electrolysis cell in the amalgam decomposer at a graphite surface with water to hydrogen and caustic soda ... 

The caustic soda solution is directly produced in the desired concentration of 50 wt%. Its purity is excellent. The high density of mercury (13.5 g cm ) enables a nearly perfect separation of the amalgam at the outlet of the electrolysis cell from remaining brine droplets. This is completed by washing with water prior to entry into the amalgam decomposer. 

Alcoholates are interesting niche products for the amalgam process. They can be extraordinarily economically produced with alcohols in place of water in the amalgam decomposer using special catalysts. This application most probably will be remaining in future for the amalgam process. 

Sodium amalgam (a liquid sodium-mercury alloy) circulates to the amalgam decomposer, where sodium reacts with water to form NaOH(aq) and Hj. 

Figure 6.19.7 Mercury process (a Hg inlet box, b cell room, c Ti anodes, d end box, e wash box, f amalgam decomposer, g Hg pump, and CW cooling water). Adapted from Hamann and Vielstich (2005).

Figure 15. Processing of hydrogen gas from the amalgam decomposer...

Figure 29. Processing of sodium hydroxide solution from the amalgam decomposer a) Vertical decomposer, b) Collection main c) Collecting tank d) Pump e) Cooler f) Mercury removal filter...

---