Common Salt & Chlor-Alkali Industries. Chapter 4

Chapter 4 Common salt & Chlor-alkali industries Chlor-alkali ("chlorine-caustic") is one of the largest electrochemical technologies —energy intensive process and is the second largest consumer of electricity (2400 billion kWh) among electrolytic industries. —The main products are Cl2, H2 and NaOH but Na2CO3 also included because it is also highly produced from brine. —The main three of them use an electrolytic cell in which electrolysis occurs when a compound is decomposed by passing electricity through it. The reaction occurring is not spontaneous, it is forced by applying a voltage. 1 Cont’d… Electrolytes: are substances whose aqueous solutions allow the passage of electric current and are chemically decomposed. Electrolysis is the process of decomposition of an electrolyte into its corresponding ions, when current is passed through in it. Electrolysis of NaCl solution: The solution of NaCl besides Na+ and Cl- ions possesses H+ and OH- ions due to ionization of water. However, the number is small as water is a weak electrolyte. When potential difference is established across the two electrodes, Na+ and H+ ions move towards cathode and Cl- and OH- ions move towards anode. 2 Preferential Discharge Theory —If an electrolytic solution consists of more than two ions and the electrolysis is done, it is observed that all the ions are not discharged at the electrodes simultaneously but certain ions are liberated at the electrodes in preference to others. —If more than one type of ions is attracted towards a particular electrode, then the one discharged is the ion which requires least energy. For example, —The discharge potential of H+ ions is lower than Na+ ions when platinum or most of the other metals are used as cathodes. —Similarly, discharge potential of Cl- ions is lower than that of OH- ions. This can be explained by some examples given below. 3 Chlor -alkali industry Use NaCl(brine solution) as a raw material to produce NaOH, Na2CO3 & Cl2 Leblanc process ✓ Common salt is mixed with H2SO4 & heated in cast iron furnace NaCl + H2SO4 → NaHSO4 + HCl NaHSO4 + NaCl → Na2SO4 + HCl ✓Breaking , pulverizing and mixing with coke of the salt cake gives : Na2SO4 + 4C → Na2S + 4CO Na2S + CaCO3 → Na2CO3 + CaS HCl + MnO2 → Cl2 + MnCl2 + H2 — MnO2 is very expensive due to this Leblanc process is commercially less attractive. 4 Leblanc process diagram 5 Deacon process ✓Oxidation of hydrogen chloride in to chlorine gas by oxygen in the presence of porous bricks impregnated with CuCl2 as catalyst. 4HCl + O2 CuCl2 2Cl2 + 2H2O Solvay process ✓ The process produce soda ash and the overall reaction is from the anions in calcium carbonate with cations of NaCl gets interchanged in indirect manner . Step1. ammonia gas is absorbed in concentrated brine. Step2. lime stone is heated to give quick lime and CO2 Step3. ammoniated brine is passed from the top of solvay tower and CO2 is flow upwards from the bottom of solvay tower. NH3(aq) +H2O(l) +CO2 (g) → NH4HCO3(aq) NH4HCO3(aq) + NaCl(aq) → NaHCO3(s) +NH4Cl(aq) 6 The Solvay process: Manufacture of Na2CO3 7 Step 4. filtration of suspension of NaHCO3(s) & NH4Cl(aq) Step5. decomposition of NaHCO3(s) by heat in to soda ash. 2NaHCO3(s) Na2CO3(s) + H2O(g) + 2 CO2(g) Step 6. quick lime is slaked with excess H2O giving milk of lime. CaO(s) + H2O(l) Ca(OH)2(aq/s) Step7. Ammonia recovery: 2NH4Cl(aq) + Ca(OH)2(aq/s) CaCl2(aq) +2NH3(g) +2H2O(l) 8 Electrolytic process Simple electrolysis: brine solution is taken in the electrolytic cell with graphite electrode immersed in it. Na+ move towards the cathode --- get reduced Cl- move towards the anode- -----get oxidized Cathode: Na+ + e- → Na 2Na + 2H2O → 2NaOH + H2 Anode : Cl- → Cl + e2Cl → Cl2 ➢ Disadvantage of simple electrolytic process is undesirable side reactions. 2NaOH + Cl2 → NaCl + NaOCl + H2O 2NaOH + Cl2 → 2NaOCl + H2 To overcome the undesirable side-reactions; Diaphragm cell (asbestos) and membrane cell (with polymer film) that separates the anode and cathode compartments are applied. 9 Diaphragm cell The diaphragm process use inert electrodes – a titanium anode electrode to withstand attack from the chlorine formed and an iron mesh cathode, which is not affected by the hydroxide solution. Advantage: • large quantities of NaOH can be produced at reasonable costs and at quite acceptable levels of purity. Disadvantages: • There is always a small amount of chloride in the NaOH produced, • There are health and environmental problems with small losses of asbestos • Hypochlorite, a strong oxidant, in the waste brine solution and this needs to be removed before the brine is discharged into the environment. 10 Cont’d…. NaOCl and NaClO3 will contaminate the NaOH product 11 Cont’d… 12 13 — A membrane cell is a diaphragm cell with an improved diaphragm called a ‘membrane’. This is made from PTFE, making it a plastic membrane, which has been modified to include anionic groups to act as an ion-exchange membrane. — This allows sodium ions to pass through it but not Cl- or OH- ions. — PTFE is very inert and can withstand immersion in hydroxide solutions. 14 Cont’d… NaOCl and NaClO3 will contaminate NaOH — No by-product of NaCl due to the membrane preventing the chloride and hydroxide ions to pass through. — It has negligible environmental impact. The advantages are much more preferable than the mercury and diaphragm process. 15 16 17 Castner- Kellner process (Hg-Cell) Uses a flowing Hg as a cathode Titanium anode immersed in brine electrolyte solution is used. Contamination of NaOH by NaCl is overcome. Cl- moves to the titanium anode where as Na+ moves to wards the Hg cathode Anode rxn: Cl- → Cl + e2Cl- → Cl2 Cathode rxn : Na+ + -e → Na 2Na(Hg) + 2H2O → 2NaOH (aq) + H2(g) 18 Advantage: it produces very pure NaOH and it avoids using asbestos. • It has a system to pump back and recycled Disadvantage: If Hg is lost to the environment, it will be harmful to the aquatic life and through out the food chain too. —However, there is an unavoidable mechanical transfer of Hg to the brine solution, which is discharged into the environment back to the ocean . 19 20 Hg-Cell Process flow sheet 21