Eco-profiles of the European Plastics Industry SODIUM HYDROXIDE A report by I Boustead for Plastics Europe Data last calculated March 2005 naoh 1 IMPORTANT NOTE Before using the data contained in this report, you are strongly recommended to look at the following documents: 1. Methodology This provides information about the analysis technique used and gives advice on the meaning of the results. 2. Data sources This gives information about the number of plants examined, the date when the data were collected and information about up-stream operations. In addition, you can also download data sets for most of the upstream operations used in this report. All of these documents can be found at: www.plasticseurope.org. Plastics Europe may be contacted at Ave E van Nieuwenhuyse 4 Box 3 B-1160 Brussels Telephone: 32-2-672-8259 Fax: 32-2-675-3935 naoh 2 CONTENTS ELECTROLYSIS OF BRINE......................................................................................................4 PRODUCT TREATMENT...........................................................................................................8 PARTITIONING ...........................................................................................................................8 SODIUM CHLORIDE .......................................................................................................................8 WATER EMISSIONS .......................................................................................................................8 STEAM INPUT TO THE CHLORINE CELL .........................................................................................9 HC L INPUT TO THE CHLORINE CELL .............................................................................................9 NAOH INPUT TO THE CHLORINE CELL ..........................................................................................9 SULPHURIC ACID INPUT ................................................................................................................9 HYDROGEN EMISSIONS FROM THE CHLORINE CELL ......................................................................9 CHLORINE EMISSIONS FROM THE CHLORINE CELL ........................................................................9 ELECTRICITY USE IN THE CHLORINE CELL ..................................................................................10 ECO-PROFILE OF SODIUM HYDROXIDE.........................................................................10 naoh 3 ELECTROLYSIS OF BRINE Over 90% of all industrial chlorine is produced by electrolysis, a process in which an electric current is passed through a brine solution. An important by- product of this process is sodium hydroxide. In its simplest form an electrolytic cell is as shown in Figure 1. Two plates, or electrodes , are inserted into a brine solution and connected to a DC power supply. The electrode connected to the negative terminal is called the cathode and the electrode connected to the positive terminal is called the anode . When the current passes, chlorine gas is liberated at the anode, hydrogen gas is liberated at the cathode and the electrolyte is gradually converted from sodium chloride to sodium hydroxide. The net result of the reaction can be written as: NaCl(aq) + H 2O(l) = NaOH(aq) + ½H 2(g) + ½Cl 2(g) All of the products of this reaction are marketable and so should be as pure as possible - hence one reason for the initial purification of the brine. Quite apart from the poor energy efficiency of the simple cell, it possesses a number of other disadvantages which call for a more sophisticated design. Principal amongst these are: (a) It is difficult to collect the hydrogen and chlorine gases, (b) It is difficult to keep the hydrogen and chlorine gases apart. This could cause serious problems because they could react explosively with each other to form hydrogen chloride. (c) The electrolyte is a mixture of sodium chloride and sodium hydroxide which would need further treatment to separate the marketable sodium hydroxide. (d) Cells operate at elevated temperatures and hot sodium hydroxide solution will dissolve chlorine. (e) The simple cell is not a continuous process. (f) Once a significant proportion of the sodium chloride has been converted to the hydroxide, the cell is operating with dilute solutions of NaCl, which makes it inefficient. naoh 4 - + chlorine cathode anode hydrogen brine NaOH Figure 1. Simple electrolytic cell. One of the first methods employed to overcome some of these problems was the diaphragm cell shown schematically in Figure 2. It differs from the simple cell of Figure 1 in that the anode and cathode are separated by a permeable membrane (originally made from porous pot). The level of electrolyte in the anode chamber is maintained at a slightly higher level than that in the cathode chamber by allowing fresh brine to trickle in at the same rate as it runs out through the diaphragm. The net result is a continuous flow of electrolyte from anode to cathode chamber but the electrolytic reaction is identical to that in the simple cell. The diaphragm cell overcomes most of the problems of the simple cell. Hydrogen and chlorine are produced in separate chambers, sodium hydroxide cannot diffuse into the anode chamber to react with the chlorine because of the continuous flow of brine through the diaphragm, the anode is presented with a constant concentration of sodium chloride so producing chlorine at a constant rate and, finally, the process is continuous. naoh 5 hydrogen chlorine - + brine NaCl+NaOH Figure 2. Schematic diagram of a diaphragm cell. From an operational viewpoint, the brine used in diaphragm cells must be carefully purified. It is especially important that the concentrations of Mg ++ and Ca ++ ions are minimised because the presence of sodium hydroxide will cause these two impurities to precipitate. This often occurs in the diaphragm itself and blocks the pores. In modern diaphragm cells the diaphragm is usually made of asbestos, the cathode is usually a steel wire mesh and the anodes are usually titanium. The diaphragm will usually last 3 to 4 months before it needs replacement. The membrane cell is based on the same principle as the diaphragm cell but the membrane is usually based on cellulosic fibres. One of the main commercial disadvantages of the diaphragm cell is the production of a mixture of sodium chloride and sodium hydroxide rather than pure sodium hydroxide. This problem is overcome in the mercury cell (sometimes called the amalgam cell) illustrated schematically in Figure 3. This cell relies on the property of mercury, a liquid at room temperatures, to dissolve sodium metal to form an amalgam which remains liquid until the sodium concentration reaches 2.5wt%. In the mercury cell, mercury flows slowly across the steel base of the cell and electrical contact is made through the base. A stream of clean mercury is fed in at one end of the cell and liquid amalgam is extracted at the other. Similarly, saturated brine is fed into one end of the cell and depleted brine is extracted at the other; this maintains a constant concentration of brine within the cell. This type of cell requires to additional external facilities; some means of decomposing the amalgam and some means of regenerating the brine. The amalgam can be readily decomposed with hot water and since the process is separate from the electrolytic cell, the sodium hydroxide is pure and can be naoh 6 obtain at commercial concentrations at this stage. Furthermore, since the hydrogen liberated during the decomposition is outside of the electrolytic cell, there is no possibility of any reaction between hydrogen and chlorine. saturated brine depleted brine anode amalgam outlet mercury inlet cathode Figure 3. Schematic diagram of a mercury cell. See text for explanation. During passage through the cell, the brine concentration is reduced by 10%- 15% of its initial value. The brine is regenerated by passing it through a suspension of solid sodium chloride. However, because of the continuous recycling, it is important that the resaturation plant also incorporates a further purification operation to prevent the build-up of impurities. In the sample of producers examined in this report, 68% were operating mercury cells, 12% were operating diaphragm cells and the remaining 20% operated membrane cells. The mix of products also varied from one operator to another. Most producers manufactured chlorine, sodium hydroxide and hydrogen. One producer however made no chlorine; all of it was converted to sodium hypochlorite. The quantities of hydrogen recovered for further use was variable, ranging from one producers who vented the whole of the hydrogen output to the atmosphere to one producer who recovered almost the whole stoichiometric amount. All operators of mercury cells also produced sodium hypochlorite. Smaller quantities of hypochlorite were produced by the operators of membrane cells but very little hypochlorite was manufactured by the operators of diaphragm cells. The production of the different products by the various routes are summarised in Table 1. naoh 7 Table 1. Production of chlorine, sodium hydroxide, hydrogen and sodium hypochlorite in thousands of tonnes by the different production routes in the sample of plants examined. NaOH and NaOCl are expressed as 100% chemical and not as solution mass. Production from Chlorine Sodium Hydrogen Sodium plants operating: hydroxide hypochlorite Mercury cells 5007 6386 189 374 Membrane cells 1836 2045 51 125 Diaphragm cells